I love with friends
How amicable! "Keep your friends close..."
We had a different policy in our flat: door open = walk in, door closed = walk away. could work for you, especially if (as you say) they're friends
I would steer away from fitting a Yale lock because it is all too easy to lock yourself out of your room and speaking as a landlord myself I have had many a late call from tenants requesting a visit with my keys, which is of course going to cost them. A deadlock will be best without the need to change the handles or a mortice lock chopped in and the new longer handle would cover the old screw holes so would still look ok.
i was standing in a lock/key supplier last year and another customer came in wanting a 3lever mortice lock fitted to a door,and they quoted supply and fit nearly £100,bejesus there on a good screw.
the FD door is probably made of rough chipboard with a smooth skin. I find chip quite hard to mortice out, it will blunt your chisels. A pro may use a router with jig which will be quicker and probably neater.
You can get a good 5-lever deadlock at Wickes or Screwfix at a low price, or a 3 lever will be cheaper, but not as good. You probably don't need a BS lock.
http://www.screwfix.com/search?search=deadlock#sort_by=price
http://www.wickes.co.uk/search?q=deadlock:price-asc
Wickes own-brand are made by ERA which is a fair make. Union perhaps is better.
Older FD door linings often have deep rebates or stops so you may need a 3" case.
If you get a sashlock it will have the latch as well as lock, but you will need a new lever handle with a longer plate to include a keyhole.
i was standing in a lock/key supplier last year and another customer came in wanting a 3lever mortice lock fitted to a door,and they quoted supply and fit nearly £100,bejesus there on a good screw.
I have rang a few "handymen" and one quoted 40£ while the other said 60£ but steep for not that much of a job.
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