Hi Tony,
Well... Vaillant Engineer came out on Saturday afternoon, I mentioned the problem, and told him which parts had been replaced. He came back in said the water was working and that he'd replaced the Diverter Valve (which had been replaced a few weeks ago), I told him it had been replaced twice already in 6 months and water still kept losing heat. He said if "He'd replaced it, it would be fine", OK we'll see…
... So, Sunday I had a a shower and every 2-3 mins I had to keep turning the thermostat up as was losing heat.
... Yesterday, I ran a bath, I had to turn the tap off half way through as water lost heat. After ten mins, I ran the tap again, water scoulding hot for first minute or so, then you can keep your hand underneath it, when bath finished running, it was at the right temperature to get in, without having to run the cold tap.
Why is it that when water loses heat, it also loses pressure??
Put the heating on at same time as bath running, it took forever to come on, this is driving me nuts now!
So... Vaillant engineer on return call tomorrow.
Keep you posted.
Emma