Wow, thanks all for the responses.
ChrisR - it was black solid stuff, so I assume that is from the boiler side.
The plumber recommended that I had a sieve fitted when we changed the boiler, which was supposed to trap any gunk in the old pipes, but each time the sieve has been opened there is no solid stuff in it, but the sieve itself seems to be 'furring up' and I am worried that this reduces the water flow which may stress the pump (?) or other parts of the boier, so thinking of removing this.
I have a combimate installed on the cold water supply, which is supposed to be a food grade water treatement which should inhibit the build up of limescale on heated water. Not sure how good this is, when I researched it before buying it I read varying reports of how 'excellent' it was and also how 'useless' it was - nothing conclusive I could latch onto. The decider was that in my house over the years, 2 electric showers packed up due to limescale build up, and in the same time, the shower we had installed at my brothers house at the same time as my first one, was running OK - and he had a combimate installed. So I put one in too! But that would not help with the black boiler-side buildup problem I guess. (Incidentally that was about 18 months ago and the electric shower we changed at the time seems to be OK so far - sounds like a positive vote for combimate).
bengasman, it is possible that the installer did not flush the old system when fitting the new one, although it was discussed as part of the job. I was not around to oversee, and the installer did not fit a thermostat as originally agreed, also left brickwork around the vent unfinished, burn marks on the wall from soldering, untidy pipework, slightly off-vertical boiler .... not one of the better tradesmen as it turned out. But the sieve does not show any bits in it, and I have had to change the Secondary exchanger twice now so I am not sure if I can really hold him responsible for this problem.
I will look at the combiguard suggestion, billy bob, but again this seems to be on the DHW side so not sure if it will handle the boiler-side issues.
Water softner might help - but again not with the boiler-side so not too sure if it is worth the hassle.
You guys seem to have covered most angles here, any more suggestions would be welcome - however could you point me to the best places to obtain the chemicals to recondition the secondary heat exchange unit I removed over xmas, I am not sure if there are any controls over these chemicals or whether I can get them from plumbers merchants (any idea of trade names for them?)
All suggestions gratefully received, thanks.