Really stupid question about a mobile phone

[LaBelleDameSansPatience]

Dh has a simple phone he hardly ever uses. It has finally run out of its initial £10 credit. He needs it on Sunday. How can he top it up? He doesn't know what network it is ... Bought maybe 2 years ago. (He really hardly ever uses it!)

Will the network provider be on the sim card?

If he hasn't used or topped it up in last few months it may well have expired. If he buys a new payg sim card (from supermarket or newsagent) he can top it up at a cashpoint.

Will that mean a new number?

Yes, it may well have been reallocated by now anyway.

When the phone is turned on is there not something saying what network it is on based by the signal strength?