@Neil
Thanks for the reply, sorry not got back to you sooner, been trying different permutations and my Garmin copy's files at a painfully slow 1MB/s!

To answer your question, I did install the full EU map, but also tried just loading the UK map too, just in case. I think I solved the problem though.
It appears that it was because I had added the ASR files after installing the map update. I'm guessing the MapInstall program amends some of the configuration files to use the updated ASR files, instead of the ones associated with my original map.
Not sure if that was due to an issue with my original installation or something that happens on every Garmin and I was just doing things in a different order to everyone else.
Either way, if anyone else has issues with the additional files, perhaps it is worth copying all the additional files to your device first, then installing the map update last.