Take some Levs to get you started (say £50 worth) and the rest in Sterling which will give you an excellent exchange rate. Notes must not be written on or torn otherwise they will not accept them. Scottish notes are not accepted in Bulgaria.
Not the best advice Roger as most people arrive at stupid o'clock in the morning so it is extremely handy if you have a small amount of local currency with you on arrival .
I managed to change some Scottish £20 notes in a Buru de Change in Bankso this year, they gave it a funny look and then handed over the Lev's. It save me paying a hefty fee on my credit card for some cash.