A problem solved.. hope it help other nubies like me. After the water in fuel light came on and seemed to stay on longer than usual I decided to drain of the water.. so far all ok. Replaced the assembly at the bottom of the filter and fitted the electric connection plug. I will just test the mk5 2.5di banana and make sure she is ok.. set off up the road thinking she seemed a bit down on power. Decided to take a run along the local motorway to see if the problem cleared.. couldnt pull the skin off a rice pudding.. so slow the police shepherded me along till I eventually reached 40 (down hill!!) big embarrassment. After much asking I spoke to a friend at a funeral. I told the story and he diagnosed both the problem and the solution without even seeing the van. Asked me if I was wearing rubber gloved when I did the water drain job ?? .. um no I wasnt ? He said put on a pair of rubber gloved and try tightening the bottom drain plug with a good strong grip of my hand (hence the rubber gloves) I easily got a good third of a turn on the yellow plastic plug. Job done no more police escorts and back to full power. Key diagnostic clue was that she started ok and didn't vibrate madly so the problem was fuel supply inadequacy at high revs..
Hope this helps someone else.