BoilerMate 2000: Making some noises, sludge & overheating

Hi,
I had an engineer out to take a look at our Potterton Suprima boiler as I had though this was the problem.
Every now and again when the Potterton would fire up you would hear an occasional noise from what I could only describe as coming from above under the floor boards, like the clattering of pipes or almost as if something was in the floor boards. That alarmed me but it doesnt happen all that much, but when it does the Potterton boiler light shines red and I have to reset it.
We had an engineer out and he looked at the boiler and said it was perfect, although taking a look at the Boiler Mater storage unit in the airing cupboard he noted that there was a build up of sludge and over heating in the hot water storage unit. I was told that under the insurance the storage tank is not covered - which surprised me.
So now i'm at the stage where I should find a reliable heating engineer which might be tricky. Before I do, is there anything I can do help alleviate any of these problems i.e drain the tank of sludge. I'm a complete newbie with this sort of thing so any instructions on how to do it would be very helpful? I've looked at the boiler mate 2000 manual but made no sense to me so if anyone can describe in a way of do this then do that I would be most grateful for any help / assistance. Thanks all.

Hi, I've opened the front panel and can see a slight leak from the copper pipe located in the centre at the base where the L shape bend is. I could also hear some ticking type noise on the right hand side pump which didnt sound particularly normal. I looked at teh date of manufacture and its made in 2002. I think I'll need a replacement thermal store now. Can anyone recommend a good reasonably price replacement Thermal Store for the Boilermate 2000 ? it will be used with a Potterton Suprima boiler which is working fine at the moment. So would rather not have to replace the boiler in the kitchen too - any suggestions ? Thanks.