OK, I'll tackle that one @RubyRubyRubyRubyAaaaah. I'll have to make some whacking great assumptions though!
My local gps is 1/49th career average salary. So I'm going to use that and assume a starting salary of £20k, wage inflation of 2.5% p.a. I'll also just assume payments are made once per year rather than monthly which makes a wee bit of a difference but I'm being lazy! I'm also ignoring management charges because it's 11pm, these would reduce both the private pension and the isa.
OK, so
25% reduced Local Govt Pension
Starting salary 20k, career average is 25545. Therefore full pension is 20/49*25545=10426, 25% reduced is £7820 per year
High risk ISA/stocks and shares
let's assume 8% return which Hargreaves class as high risk.
That would give you a fund of just over £74k, and an annuity of £3260
Private pension
Assuming you got a return of 4% per year on average, plus 20% tax rebate, you would have a total fund of about £46500 at the end of the 20 years. Using the annuity figures I was looking at earlier for a 62 year old woman that would buy you an annuity of about £2k ish per year
You could go high risk within a pension which would get you more, but you're still a whack away from the career average.