hijet van 1998 running really rich


I have a 1998 hijet van which i have painted in the style of the scoobydoo mystery machine, with all the relevant stickers etc.
Problem is I can't get it through its mot due to the emissions being off the scale. Its had a new exhaust and cat(ouch), new lambda sensor, new(used)ECU, full service etc, new thermostat.Its got to the point where it would be too expensive not to fix it the amount ive spent.

My mate whos a mechanic suggest the cold start sendor, trouble is can t locate it??, Any one got any other suggestions??
Its the 993cc EFI version.

Please help me .....sob
I love the van and want to further convert the inside, but only if I can drive it lol