To get the water hotter you need to reduce the flow across the heat exchanger. Think of it as a fixed amount of energy going into the water ... if you increase the flow the energy is still the same but as it is in a greater volume of water the water will not be as hot. It sounds like the effect of flushing the Wc is reducing the flow of water over the heat exchanger. I am assuming that your shower is not a thermostatic shower ? ( if it was you would expect the shower temp to stay the same but the flow change when other hot or cold taps / wcs are operated).
It is possible that you have never seen this problem before and this may be down to the current weather. If you have normally good supply pressure and flow then the effect of flushing a wc in normal circumstances will have negligable effect on the volume of water flowing across your combi heat exchanger - however - if you are experiencing freezing weather like most of us it is possible that your incoming water supply is partially restricted possibly by supply pipes starting to freeze ( thus reducing the bore size and reducing flow) but could also be that your stopcock is not fully open ( open anticlockwise fully and then come back 1/2 turn)
The fact that you say the water is only lukewarm is probably not suprising if the supply is coming in at close to zero degrees as the boiler will merely add heat to the water so if the incoming temp is 10 degs below normal then the output will be a similar amount below normal - obviosly by turning the hot flow down ( so long as it is sufficient to activate the combi) you will get hotter water. Also make sure that if your combi has a water temp control that this is turned up.

Hello Scotty,
Many thanks for the reply and information above, it was most useful and you are correct.
I slowed the flow down and yes the water is hot, but was getting difficult to have a quality shower. I've checked the supply pipe coming into the house and the pipe are quite exposed in a very draughty part of the kitchen behind the units. The pipe had frost on it but water was flowing through. I have spent a good part of the day re-lagging pipes and ensuring they are all well protected.
I have had a shower tonight with the flow slowed down but it is a lot better than what is has been for the last few weeks. You have probably saved me a potential mains water burst too with this weather we are having.
Many thanks for the advice it was top notch.
Harvey