Tam, an alternative: when my passport expired here, I went to the American embassy. They renewed my passport, which I then posted off for a new stamp. No problem, all you provided was your old passport and indefinite leave letter.

Last year my passport expired again. I sent it off to the American Embassy and got a new one, but found out the UK government now charges a substantial fee to put the new stamp into your passport. HOWEVER, if you travel with both your new passport and the old one with the stamp in, you're absolutely fine.

No queueing, no horrible toilets, no fees, no tests!

However, if you decide to take the citizenship exam, best of luck.