Manual states 10w40 and think Haynes manual has some tie in with Duckhams so they recommend QXR.
Had my Valver for 80k miles and been through a bit of oil in that time. Duckhams QXR is what National Tyres use and they are cheap. Never had a problem with that and they used to do change inc filter for £15 but won't touch Renualts any more due to filter ocmpatability problems with their suppliers.
Halfords own oil is also OK. Mobil 1 not that good. Total Quartz 7000 from GSF good. Think people say to avoid Magnatec too on Valvers.
Best I have used is probably Castrol RS and Silkolene Pro S. What is more important, especially this time of year is cold start protection as that is where most wear occurs. Hence go for a 5w40. Your 15w40 won't protect as well when it is cold. I'd save that for summer use and even then a 5w40 will still work better at protecting the moment you turn the ignition.
I'd get the rating of oil right and then worry about whether you want semi synthetic or synthetic. Guess it depends on how the engine is now, milage, wear etc.