Re: B&Q Satellite - Ross freesat box and dish
If you live anywhere but the top half of scotland then you should get a good picture even in heavy rain. Sounds like you're still slightly off with either your dish or the LNB skew. A good lower noise higher amplification LNB would help but not as much as a simple dish adjustment. Remember that tiny movement can make the difference between a poor signal and a great signal.
As for standard twin LNB's they all tend to be the same specs so shouldn't make much difference until you pay a lot extra for higher specs.