forester vote fore me here,
how many miles do you cover a year ? if not a great deal and used for short trips i'd go petrol as there seem to be a few people with dpf issues, (same of any diesel)
as for parts yes they are expensive if you only use main dealer, theres been a couple of times when i've had to put the phone down due to laughing so hard whilst talking to them, but the internet is a brilliant place to get replacement bits and there are loads of companies that specialize in jap/subaru parts
as your in sheffield, may be worth giving Martin a ring down at sheffield subaru sdervices to give you and idea of service costings and what not, great independent with a wealth of knowledge and good bloke to boot