DaJJHman wrote ...

seeing as I don't know your specifics on design, I thought I still might throw this out there: Doped Electrodes. You can buy, at least in the US and surely in some hardware store of the UK or eBay, Tungsten rods that have been doped with Thorium to make spark forming much easier - then there are doped electrodes used in Marx generators that have higher efficiency (I know you are not making a Marx, but it might help). If the hardware store welding rods don't help, maybe you could get some from a specialized supplier online, or even eBay as I have seen triggered gap assemblies fairly often. You could possibly even make some yourself if you wanted, but I do not recommend that because you could form nasty vepors with the radioactive source
these are not even as radioactive as your smoke detector most likely, but still might be regulated - I am not familiar with UK laws, but in the US they would be classified as scientific/industrial/exempt quantities because there is actually so little material in these electrodes

just my $0.02

-Jimmy