I know that lots of plants are described as "poisonous" when in fact they
just give you a bit of a bad tummy, whilst others can be fatal. I have small
children so am trying to rid the garden of anythign fatal

Most of the online lists are targetted at the US but are still useful.

Google +plants +toxicity gives several useful sites. This one isn't too
bad:

http://envhort.ucdavis.edu/ce/king/P...nt/Tox-COM.htm

Some include toxicity to commercial livestock as well.

It probably is worth avoiding some of the really deadly poisonous
plants if you have small children. Some colchicums for instance can be
lethal in very small doses.

Regards,
Martin Brown