Additional instantaneous hot water units connected to combi boilers

Hi
I have a Worcester 24i RSF Combination boiler which is wonderful for giving us hot water and central heating in our flat. The combi is connected to the mains (self installed 25mm Diam feed from the road giving a fantastic flow rate of 20Litres/minute free flowing, less going to the boiler in case you are worried)
However during the winter months when the mains water temp is down around 5degrees. the combi struggles a bit and the hot water is not as hot. Now I enjoy hot fast flowing showers to wake me up in the morning. I have a thermostatic mixer unit off the bath so the temp doesn't drop when say the wife flushes the toilet. During the summer we have mighty powerful showers as the mains water temperature is high and thus the combi heats the water up by 30 degrees and gives a good flow rate of 9litres/minute on pure hot water so when the thermostatic shower kicks in to add a bit of cold we get 12-15L/m. So my simplistic thought is why don't I add an electronic instantaneous water heater just before the mains feed goes into the boiler. that way the mains water is heated to say 15-20 degrees as it is during the summer months and hey presto I get a might good shower again. All I would need is a low rated unit say 4kW to raise the temp of the mains water 5 degrees or so.
I have rung a couple of companies who do water heaters and all they say is oh no, you cannot do that. Why I ask?. Because you can't is the answer!!!
So questions are Is it possible to do what I propose? It is logical? Can anyone advise on a good 4KW water heater. I do not think i can have a storage type unit, its has to be the instantaneous variety?
I have also thought about putting the heater next to the shower unit but a lot of heaters do not like taking in hotish water to make it hotter! I have also though about heat trace tape where it keeps the water in pipes at a ertain temp. i am await info on this but I suspect it will be lots more expensive than my water heater idea.
Looking forward to hearing from you
Regards Vince
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Well, it would turn your 24kW combi into a 28kW one. It might have been better to install a 28kW in the first place. It would give you an extra 16% water flow, at the cost of a lot of electricity.
Christian.
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