You will usually get 40 days grace (depending on the type of domain [.co.uk is the best]). So unless it expired a long time ago you should still be able to retrieve it by paying the renewal fee.
There are a couple of tools that you might be able to use (depending on how long ago it expired)
DNS Tools - try typing in your domain name and then look for: 'registrar of record'
Traceroute, Ping, Domain Name Server (DNS) Lookup, WHOIS - type on your domain name and get info on who owns, and who registered your domain name.
Registrar of RecordIf this fails