I've purchased a 2003 4.4 V8 for £1700 which is sitting low at the rear and seller thinks the compressor is worn out.

How simple is it to pump up the suspension?

Is the compressor in boot or underneath and which line off the dryer do I need to pump up?

Compressor is under the spare wheel in the boot..it can be a manner of things that cause it to do that. Burst bags, compressor, valves , height sensors ect...I would get it on diagnostic first and check resovoir pressure and check all the valves are opening and closing as they should , this should give you an idea to where to start fixing things..