Hi Falesh, 

If I were designing your system then firstly given your location my design outdoor temperature would be more like -3/-4 to achieve 21 degrees throughout your property. Given that you are a 1965 bungalow I would be cautious of your walls, unless you know for sure what the cavities are filled with (if anything). 

With the above in mind your projected heat loss may not be unreasonable, but best to be sure first before going any further with sizing.

Often bungalows are piped from the loft, are you sure yours is not? With regard to potential leaks, if you currently have a sealed system (combi boiler) then you would know, as your pressure would constantly need to be topped up. If you have a tank fed system currently, and pipes buried in concrete floors then that could potentially be a problem. Given that you mentioned lifting up floorboards, I am guessing that you do not have pipes buried in concrete.

In short there is no 'easy' way to find out pipework size without further investigation, but having 15mm pipes going to all radiators and 22mm leaving the boiler is a good sign so far.

From your floor plan it would likely be necessary to add a radiator to your kitchen in order to ensure MCS compliance, meaning that some work would be required to the pipework to add this additional radiator, and an installer should check pipe sizing (and make appropriate upgrades) as they are doing the installation.

It is only the main 'skeleton' of pipework that needs to be verified, meaning that you have the central part in a larger diameter, splitting down to smaller diameters to pick up radiators as it gets further away. I would deem your property low risk and it would likely need minimal alteration in order to get a well performing system.

like this (very expensive, a pig to work with, but whats needed)

Yes, very similar to that and it was hard work to get into place. 

I put a temperature gauge on the pipes last night, measuring the flow and return to see how much the temperature drops between the anti freeze cycles, and it dropped about 1 degree in an hour so when the pump is running the loss will be negligable. Even at 14 ltrs/minute the water won't be in there very long, although the pipe is about 15mtrs long so there is about 30 ltrs in the two pipes that are underground.

Thanks for responding

@editor, Can the pipe be too large? We have 32mm to our HP with a 5kW heapt pump.  Apart from the cost and the amount of water that is in the pipe losing heat, is there an issue with the pipe being too big?

The only disadvantage of a bigger pipe is that the water will take longer to move through it, so has more time to lose heat.  On the other hand there is less (as a proportion) exposed to the outside so the rate of heat loss per unit volume of water is less.  Im not sure which effect is greater, but I wouldn't worry about it assuming the pipe is well insulated.  If you have proper, professional, preinsulated pipe with closed cell insulation (so it doesn't get wet) like this (very expensive, a pig to work with, but whats needed) then you should be fine.

It's a new build and the pipes to the HP are under the slab. They were put in very early on and I was given the spec of a pre insulated pipe to order when we were digging the foundations. I had very minimal understand of what was needed for a heat pump at that point and thought builders etc. knew what they were talking about. 2 years further on I know to question everything in much more detail.