(Multi-part post):
The nearest Spanish airport to San Sebastian is Bilbao. There are no services from Manchester. You clearly don't want to travel on the Iberia service from Heathrow but would you consider flying from Stansted? If so, there are cheap easyJet flights from there. (According to the Bilbao Airport website, those are the only UK flights to Bilbao):
http://www.easyjet.com/en/book/index.asp
Buses run from Bilbao airport, into the city centre, every 30 minutes. Or you could take a taxi; it's a 15 minute journey, for around �20. However, despite the geographical proximity, the rail journey to San Sebastian takes around 5 or 6 hours (with a long wait at Miranda de Ebro).
Hiring a car would be much easier. It's about 100km, with (according to the ViaMichelin routefinder) a driving time of little over an hour. Hire car options are here:
http://www.aena.es/csee/Satellite?cid=10476584 32601&pagename=Estandar%2FPage%2FAeropuerto&SM O=3&SiteName=BIO&c=Page&MO=3
However, if I can persuade you to consider flying from Stansted, it might be easier to forget about flying into any Spanish airport. San Sebastian is just over the French border and only 50km from Biarritz, which is served by Ryanair:
http://www.ryanair.com/site/EN/