The filter itself is about £25 plus fitting

I had the same sort of problem on my 4.6, got it serviced £160 now it works fine, guy said the filter for the LPG was so full of crud the system just couldn't feed enough gas into the engine unless the revs were up.

Had the same problem on my drivers window, got a replacement mechanism of ebay for £42 incl p & p. Took about an hour to swap it over and it works fine now. Just like to say thanks for all the advice as I can now drive at more than 50 without having an anoying whistling noise.