just a thought that was running through my mind tonight, do you loose bhp/torque before a service and gain it back again after a service? if so how much or percentage do you loose by the time its due a service?

Depends tbh. If your air filter is blocked up then you will be losing power prior to the service, same if the spark plugs and leads are worn out. How much depends on how badly they're worn, and it could well be noticeable.

cheers, the car seems a bit smoother with a clean filter now. i just wondered if much power is lost on anew car thats done 18k on the original sparks and oil.

I think it's fairly common knowledge that the 1.9 diesels run a lot better after the first oil change...