Most of the information you required can be found here.
http://www.direct.gov...gFrom1998/DG_10034867
..... but in short
The threshold for repayment is an annual salary of £15k. Above that you pay 9% over that figure back.
Interest accrues on the loan all the time since you had it but at relatively modest rates, tied notionally to the inflation rate at the time. Currently no interest is accruing (because inflation rates are low).
You seem to have slipped through the net; application for the original loan and hence repayment is tied to your NI number. I'm surprised you have managed to 'get away with it' this far. Can't comment on whether that will continue.
There is an obscure rule somewhere in the 'rules' that says if one hasn't paid it all off after 20 (I think - someone will correct me if not), the outstanding balance is written off. This rule applies, I think if one of the following applies: -
a) you are outside the jurisdiction of the UK tax system - i.e. you emigrate
b) you never earn enough beyond the threshold to pay the loan off.
Whilst the former may have applied to you in the recent past, it doesn't apply now. So I doubt that your loan would be written off after the requisite number of years if you are evading payment. That last bit is just my opinion.