i have fitted the dump valve and sounds ok but i have found that it stays open i little bit and this make the revs drop so the dash lights come on.
its only for a second and has cut out once which is no good as i have a turbo timer fitted ive looked for leaks and found none and ive changed the vac hose to a better one but still it doesnt close any ideas.

Deffo sounds like a Dump Valve leaking problem.

What make is it? Is it a piston type? Maybe try a stiffer spring / streach the spring in it already

sorry dont know the make (piston type)
i have put the stiffer spring into the unit and still the same i have noticed that the rubber O ring makes a tight fit would it help if i greesed up the O ring?

there are two O rings one at the base and one half the way up.

Kinda kard to tell from the pic in you MMC topic what type of valve it is, but theres a good chance it may not be the right type for the car and hence your getting an air leak causing the low idle problem