Ravenheat boiler problems

I have a Ravenheat combi boiler - not sure of the model number, but can dig it out later if relevant.

A few issues which may, or may not be related...

Ever since we had it installed there has been quite a long "lag" between turning on a hot tap and any hot water coming through (although the boiler would fire up after only a couple of seconds) - sometimes up to a minute or so. I tried turning down / up the water temperature, but it made no odds.

Then, in the last few months it's got a lot worse, so much so that unless I switch the central heating on, no hot water will come out upstairs, and even in the kitchen, switching the hot tap on only seems to fire the boiler up intermittently.

Finally, in the last few weeks, pipes leading from the hallway up throught the floor into our bedroom have started knocking really loudly whenever the central heating starts up or shuts down. It's got so loud it's waking the baby every time, and frankly I get little enough sleep as it is!

Please, can anyone help?

Is it the LS80-LS100?
The best advice I can give is to get a new diverter valve.
Was the system flushed properly when the combi was fitted?
How old is the boiler?

Yes, it's the LS80-LS100 - it was fitted when we installed a new kitchen and downstairs toilet 2 years ago.

I've had a few suspicions about the plumber used on the job (although I was assured by the project manager he was CORGI registered), especially after finding he'd attached the hot water to the new toilet cistern! He told me he had flushed the system, but as I wasn't there at the time he may not have bothered.

We live in a 3 bed victorian semi that has been botched a fair bit before we bought - complete mish-mash of different radiators etc. which I guess probably doesn't help matters.

Is a new diverter valve easy (for someone who knows what they're doing!) to fit?

Any divertor valve can be difficult if you don't have the right tools. I find a self adjusting tap spanner gets me past most obstacles when changing them.

About the same time April 2005 I fitted one with the same symptoms. Called Ravenheat out and the guy fitted a new diverter which did the trick. I did a post mortem on the one that came out and couldnt find anything wrong with it. Dodgy batch perhaps?

While you are at it change the CH thermistor