To try to use an English or Scottish residential tenancy agreement in Bulgaria would be singularly unwise. Any Bulgarian judge who comes across an agreement purporting to grant an assured shorthold tenancy under the provisions of the UK Housing Act 1988 is likely to start chewing the carpet with rage, I should have thought.
There is a Bulgarian one here, with copious notes which would repay careful study along with the relevant provisions of the ЗЗД (ЗАКОН ЗА ЗАДЪЛЖЕНИЯТА И ДОГОВОРИТЕ - Law of Obligations and Contracts). http://contract.bg/content/view/28/58/
Obviously prime points for the landlord to consider are whether the tenant could in any circumstances achieve security of tenure beyond that provided for by the contract and whether it is possible to impose restrictions on subletting.
A case for local legal advice, I would have thought.
Fair dues Gimlet, there are of course all of those risks involved and why I suggested a little 'poetic license' in the preparation of a suitable tenancy agreement. If the true owner 'never' signed it then, I assume it wouldn't stand up in a court of law should the tenant wish to outstay his/her welcome. At the end of the day, it is only to suffice the Bulgarian immigration authorities that you have a place to stay whilst in Bulgaria. As far as I understand EU regulations, Bulgaria doesn't even have a right to throw you out even if you don't or, even if you don't register yourself within the 90 day limit although, there could be some fine in line with what a National would receive for a similar offence.
As you will see, the OP is pretty stumped for everything without that 'Short Term Residency' card as he can't even get himself a contract with Bulsat unless someone in that thread has come up with an answer.
I would assume that even if the 'owners' gave some sort of letter stating that the OP could stay in their house for one year then that in itself, would form some sort of binding contract if all went sour?
At the end of the day, download, edit out all the UK stuff as required, buy a company stamp, they cost a few Lev, stamp it, sign it, get it translated, submit it and if it is accepted, problem over, if it is not...... what could be the worst punishment for forging a document that was only meant to prove one had a roof over one's head?
Non EU citizens are doing it all the time with Hotel headed stationery to obtain Visas.