It's only a problem if you are needing to use the services of an immigration office. If you are just on holiday, not extending then sort it out at the airport.
Otherwise it's report to the police, get the paper report. Take to inmigration, you will be given a new TM6, the number probably crossed out, and your old number written on.
The TR60 is a much more flexible visa than the STV. Doesn't require insurance for extension, no prebooked accommodation, and can change to other visas. STV, needs full insurance for total trip, prebooked accom, can't convert it to anything else. STV is only good if you want longer than 90 days and will go home straight after, ie not need to convert it