I have a A3 2.0 FSI and need a service, looking at Audi the service price difference between full and interim is £150. The services are the same accept for the following listed below.

Air filter clean or replacement (if necessary), Fuel filter (diesel only) or Spark plugs (petrol only) replacement

Do you think it's worth £150 extra for them to maybe replace the Air Filter and spark plugs?.....
What about the spark plugs do they definetly change the spark plugs or is it only if they deem it necessary?

Air Filter, is that the filter that is located behind the glovebox? if so ain't that about £15-£20, looks simple to enough to replace.

Spark Plugs on the other hand I'm not sure they would be so easy to replace but I see Bosch Spark plugs from ECP are £10 each, so £40 in total (I just had a quick look on ebay and I see there's some Bosch x4 for £15, good deal if real).

Also if I do go for either service and bring my own oil will they reduce the price, as it's there fixed price servicing. £309 for full and £159 for Interim.

I checked Russel Automotive Centre for a full service and I think they were only £40 cheaper than Audi. I would imagine that it's better to pay the £40 extra and go to Audi as atleast I'll have an Audi stamp.