Wow, thanks for the prompt and informative replies guys. I think I will have a go at the circuit you show. What does 'U2 ' and 'U3' mean?, And I guess TL/LM 431 is the transistor part number . ( Im getting eveything from Maplins ..Im in UK btw).
The diagram of the MAP sensor you show refers to some numbers but I guess I can ignore all that ?.I have the MAP sensor already as is part of the car.
One final thing I found another solution surfing the web, that being to use a (say) 3.6 volt 0.5 watt Zenor diode bridged across the signal and 0v wires. Although this gives no adjustability , is it regarded as a good way of doing it? I dont want to fry anything or get a slightly non flat line after 3.6 volts.
Thanks again.