Having searched online for suitable 5W -30W longlife oil to top up between services I called my local dealer Hortons of Lincoln. 1 litre of Castrol was £14.99 however 5 litres of Quantum was £36 which I duly bought. I know some stores have occasional sales of suitable oil but I think this is very competitive and for once I thought I'd give a shout out for the dealer.

ASDA have Castrol Edge 4 litre for £35.00 available on line - http://direct.asda.com/Castrol-Edge-5W-30---4L/001022050,default,pd.html

Also in most stores.

This might help:

Smith and allan - 5w30 507.00

Opie Oils I've seen advertised on car forums regularly and they always seem to have some offers going on.