Despite using a chainring nut key at the rear to stop bolt from slipping and the correct 5mm park tool to remove the bolt on the outer chainring I've managed to roundout the bolt….so what now? Do I have to drill out the bolt from back of the chainring carefully? Hoping some of the bike boffins on here can offer some advice.

Drill it out from the front.
You only need to take the head off the bolt then it'll push out through the rear.

Thanks singlespeedstu