Engine running on is caused usually by RETARDED TIMING or a WEAK mixture.
I'll put my money on timing retarded by a lot, and I'll also wager that someone has set the timing and forgoton to disconnect the vac advance whilst they were doing it. Garages are ofter guilty of doing this especially if they are unused to tuning classic cars. (check your dizzy is advancing propperly as well, could be faulty)
Just my two penneth.
P.S. get the timing set propper and mixture set then go for a blast, that'll burn any exces carbon deposits off for you, don't worry about it.
Forgot to say why it runs on without ignition on.... Exhaust valves get red hot and ignite the mixture..
Timing is dead on, as per haynes manual (10deg advance@1500rpm with vacuum pipe off) i know because ive only just set it (can i have your money? 
), if there is a better setting please recomend.
Weak mixture meaning i need to richen it up a bit?
Thanks
Chris
Not havin me money yet,
Check plugs, they should
not be ashen coloured, this is too weak. They should be light grey idealy, modern engines can run weak but not a Mini.
Timing sounds OK but is it advancing above 1500, try and check with the vac pipe off and see how far it advances, you can go 30 to 35 degrees as long as the engine dont pink and it's ok lower down, depends on yer dizzy and state of tune.
10 degrees at 1500 is ok but if it is advancing at 1500 then that is not enough, check it at about 3000 and 4000 if you can and see what the advance is.
Is the vac advance working ? this would cause retarded timing at part throttle openings if not. Pull the pipe off at idle and the engine should slow down.
Worth mentioning that the needle may be weak mid range.
Rich will NOT cause running on, weak mixture causes that together with retarded timing, the exhuast valves get hot as the mixture is still burning when it exits the exhaust valves and overheats them.
