Thanks for all the replies chaps.
Before I do anything I'll try and get a macro shot of the lock and post it here.
Notes:
# The steering wheel is
not locked - it turns from one stop to the other
# The car is on blocks - all 4 road wheels are off
# None of the 4 key blades are damaged
# Two of the keys are new - they replaced the original two which have duff fobs (the plastic/rubber bits fell out of the casings)
# I've only been using one of the new keys to unlock/lock the doors while I've been doing the rear end refurb
# None of the other 3 keys have been used recently (apart from yesterday)
# The key blade (any of them) goes in about 5mm then stops - it feels like it's up against something pretty solid
I've never seen anything like this before.
The ignition was working perfectly up to now - I started the engine just a few days ago and nobody has touched it since (apart from me when I put the key in to confirm a flat battery).