Typically a locksmith will charge a £55 call out for up to half hour, then £20 for every half hour after that

Doors that latch themselves without a key can normally be opened without a change of lock

Those that only lock by key normally have to be drilled out.

The typical cost for a new lock would be £35

Time to ask a friend to hold a key for you is NOW.

Information above supplied by Halls Locksmith when I enquired by phone Cambridge 01223 416000
and then gave me the price below by email as

Saturdays and up to 9o/c at night its £60 [plus vat]
Sundays and after 90/c its £70 [plus vat]