To answer this properly you need a heat loss calculation.  Then you can work out how much flow each radiator needs which will tell you what diameter pipe (internal diameter) is required.  Until you have this its somewhat impossible to tell for certain (but see below).  Did your pervious quote include a heat loss calculation?  Have you actually established that re-piping is necessary.  Some installers love to do it but its not always necessary.  If you post your previous quote perhaps there are some deductions that can be made.  

A few more questions which might help point in the right direction, answer those you can!

• Whats your largest current radiator and what type (eg 21, 22).  How many do you have?
• Does the microbore all come back to a manifold and if not how is it connected?
• What diameter are the primaries - the pipes leaving the boiler
• Whats your house and construction/age/standard of insulation
• Whats your annual heating gas consumption and pattern of heating

You should be able to do 750W comfortably through 8mm id pipe without impacting efficiency so if no rad is  over this size no problem.  Even if they are you may well be able to get away with it with a bit of finessing of pumps/DT etc., or just replump one section.  Personally I detest throwing out perfectly good kit and a full replumb is pretty disruptive.

@jamespa Thanks for taking the time to answer.  I wasn't looking to check whether or not I needed to replace the 8mm microbore.  As you say that would require full heat loss calculation plus much more detailed work checking existing pipe runs.  Access to the pipes is difficult and I only want to start lifting carpet and cutting holes in my floor to check how far the primary pipework runs when I've decided to proceed even if it transpires that I'll need a full repipe.  If I discover I can achieve good efficiency without repiping then that's a bonus.  Additionally as the system is over 30 years old I would be concerned about possible future blockages as corrosion builds up over subsequent years.  Whilst I've had the rads cleaned out, I was warned off power flushing microbore as it can force sludge from one part of the system into the microbore. 

So at the moment I'm trying to run my regular boiler at as low a temperature as I can to lengthen its lifespan whilst I get a better understanding of the ASHP world and inistallers near me build experience. One of the new (or at least new to me) areas was the use of MLCP in place of copper pipe.  So I was trying to get an understanding of how it is used and what are its advanates and disadvantages.

Posted by: @iotum

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.  So I was trying to get an understanding of how it is used and what are its advantages and disadvantages.

OK, TBH I dont know enough about MLCP to comment with any authority.  The simple message so far as I am aware is: Nothing inherently wrong with plastic pipe but it has a smaller internal diameter and is rougher than copper pipe so for any given external diameter you get less water through.  This may or may not matter depending on your design.   Easy to route round gentle bends so particularly good in a new build where everything is 'open'.  Not recyclable whereas copper is.  Others can hopefully give a more authoritative answer.

Posted by: @iotum

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Additionally as the system is over 30 years old I would be concerned about possible future blockages as corrosion builds up over subsequent years.  Whilst I've had the rads cleaned out, I was warned off power flushing microbore as it can force sludge from one part of the system into the microbore. 

Dont get this.  If you shut down sections sequentially so there is only one route through at any given time that cant happen.  Something of a PITA but less so than taking up all the floorboards!

I note your floor area is 120sq m but its single storey.  Assuming you have decent loft insulation and the walls aren't all solid then I would doubt its 7kW and, if it is, you may be better investing in insulation upgrades instead of re-piping!  If you know your annual gas consumption in kWh then divide it by 2000h, and separately divide it by 3000h.  The loss is highly likely to be between these two figures.  eg 18000kWh annual consumption is likely to translate to somewhere between 6 and 9kW.  As a concrete example my house was 18-20kWh, measured (not calculated) loss is 7kW.

Posted by: @iotum

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So at the moment I'm trying to run my regular boiler at as low a temperature as I can to lengthen its lifespan

Thats a very good idea anyway as it lengthens run times and means its more likely to condense, which both reduces consumption and improves comfort!"

Posted by: @iotum

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I had a quote a couple of years ago for fitting a 7kW ASHP and complete copper repipe which came to over £20k for a single storey semi detached cottages (123 sq metres)

Sounds a lot, is this after grant or is the real price £25K? 

When you come to proceeding do shop around and do fell free to ask for help here.  There are good installers and some who are still using practices from 5-10 years ago.  Under no circumstances allow anyone to fit a buffer, low loss header, or plate heat exchanger between heat pump and emitters, there are unnecessary particularly in a property of your size, they compromise efficiency if not properly set up (which they rarely are) and make fault diagnosis difficult.  A 2 port volumiser is OK however and may be needed.  You want exactly zero external controls and certainly no 'smart thermostats' like Hive etc.  TRVs are OK if they must and can be useful to limit (but not control) temperature in rooms with high thermal gain or cooking, BUT you will be setting most of them to max or even removing the heads so they have no effect.  This is pretty much the polar opposite to what we have learned (in many cases wrongly) about controlling boilers!)  The controller for the heat pump needs to be sensibly sited in a representative room NOT in a cupboard.  Some have wireless controllers which you can put where you want which is probably the best.