I would suggest it is reasonable that your landlord furnishes you with a set of spare keys to your flat. If you point out the inconvenience and expense of someone from the letting company coming to let you in if you lose your one set of keys, or the expense in replacing the lock, then they should see the sense of providing you with at least one spare set. Failing that, enquire with a reputable locksmith about the circumstances under which you can have a copy key cut - maybe providing proof of ownership of the lease may help.