[Matth]

3 is poor for occasional use, due to monthly expiry of credit - a return to the "bad old days" before Orange set the cat among the pigeons with "there's no ouch in our voucher".

Discounts?
Vodaphone give 10% extra when you top up £15 or more.
Orange's "repay our prepay" is good for heavy users .. Use £10 in one week and get a free £5 for that week.
Virgin have a discount suitable for heavy users as well.

Easymobile's opening offer - 6p/min any network, any time is pretty good, and the subsequent rate of 15p/min is good for any network on PAYG, but poor for same network or landline unless you never get beyond the expensive first 3 or first 5 minutes on the other's common "step" tariffs.

If all the mobiles you are likely to call most are on one network, then you really need to choose the same network - if you call a lot of mobiles on any network, Easymobile looks the best bet, but about the worst if you mostly call landlines.

Virgin's 15p/min for the first 5, 5p/min for the rest of the day, is one of the better tariffs for same network and landline.