Posted by: @powerade

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I have no spare space in the house for a water tank other than if i use one of my existing kitchen cupboards to have it in OR under my stairs space which is not much higher than a washing machine, is that possible?

Typically DHW tanks are cylindrical and ~1.4m high (or more)

Is the loft a possibility?

If not then with the relatively small space available you may have to make some compromises on the amount of water you can run off without a reheat, or on the running cost by heating to a higher temperature than you would otherwise do.  There are solutions to this problem but they do involve some compromise (as does almost everything in life!).  How much DHW do you use?

Posted by: @powerade

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Also as my homes garden is north facing, the only outside space I have for the pump itself is fully shaded and never gets direct sun. Is this also and issue? (I will not be able to install the pump at the front of my property due to my parking)

No it doesn't make a material difference. Heat pumps draw air in from the surroundings they dont rely on solar radiation falling on them.  All other things being equal a sunny place has some advantages over somewhere North facing but not enough to exclude the latter.

I assume you don't have any useable loft space for the DHW tank?  We installed a Mixergy 210l in our loft when the Ecodan was installed 18 months ago and it has worked well.

Failing that, have a look at the Heat Geek mini heat store - this video explains all.  It should fit under the stairs but has limitations as described in the video.

@jamespa thanks. Unfortunatly my loft is converted and there is no space.

We currently have a 35kw gas boiler but even that's not enough to run any hot water tap at same time as a shower without the shower going cold. Not sure that means a low capacity tank is OK.

Also a thermino heat store is a possible solution. 
https://sunamp.com/en-gb/hot-water-solutions-thermino-range/

I think @toodles has one?

Posted by: @powerade

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We currently have a 35kw gas boiler but even that's not enough to run any hot water tap at same time as a shower without the shower going cold. Not sure that means a low capacity tank is OK.

Thats not directly relevant because you are comparing real time heating with stored hot water.  Real time heating is dominated by the raw power you can deliver to the tank (which will likely be less than 35kW).  With My 7kW heat pump and stored hot water I can run all the hot taps and a shower simultaneously, but of course eventually I run out of stored water.

For stored hot water what matters more is how many showers (or other things using lots of DHW) you have in quick succession.  This determines the size of tank you need.  If you can give some indication of that we can point you in the right direction

Posted by: @bontwoody

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Also a thermino heat store is a possible solution

Good point, I had forgotton those; this maybe a good use case.

@jamespa usually 2x 10 mins showers in morning, separated by 30mins. And same mid evening. That's on average.

Posted by: @powerade

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@jamespa usually 2x 10 mins showers in morning, separated by 30mins. And same mid evening. That's on average.

OK

Morning and evening are far enough apart you can reheat in between, so we can consider one alone.

2 * 10 min showers consume the following

@20l/min 400l

@10l/min 200l

@6l/min 120l

If you store water at 48 and shower at 40 you get about 25% more yield than the tank size, if you store water at 55 and shower at 40 you get 50% more yield than the tank size.  

You can get a 125l cylinder which is just over 1m high (you will need some additional clearance for the exit pipework.  If you store at 55 will give a yield of 180l, something like 8l/min for your showers which you may find acceptable with a water saving head.  There are also these  which are specifically designed for small spaces.  Im not sure they really offer much advantage over a small cylinder though unless access for pipework is difficult.

You could squeeze a bit more out of a small tank by turning on the immersion heater and heat pump simultaneously during draw off, so you have a 'fast' reheat.  In the 30 mins between showers you can hear 114l from 10C to 40C with 8kW - 3kW from the immersion and (say) 5kW from the heat pump.  This might give you ~280l in total from a 125l tank.

Otherwise its Sunamp and their Thermio heat batteries, which use a phase-change material to get greater storage density than water can achieve.  The 210l model will supply 300l water at 40C, which should suffice for your needs.  Its 870mm high so would fit in the space of a kitchen cabinet or under the stairs.  They are rather expensive and very heavy, but if compact and high yield is what you require, then it is probably what you need.  Its also the (relatively) straightforward option.

In summary its do-able, the best solution depends on your requirements.