"The £590 ought to be increased annually by 3% - that's pretty standard in pension arrangements,"
You must be joking!!!
Really jenny you don't needd to get too bogged down with the details. It's fairly easy to take a stab at which is the best deal in absolute terms. However it would only be a stab because there are too many variables which you have not a hope in hell of forecasting correctly: the rate of inflation and, most importantly, how long you will live are the most critical.
So your decision depends on whether you'd like £8,400 now (of which you'd probably lose 20% to income tax leaving you with £6,720) or whether you like just under £50 per month (again, if taxed, just under £40).