I should clarify, my partner didn't treat me, rather recognised what was happening and decided that going to a clinic and getting some prompt treatment was a good idea. 70 euros, some drugs and O2 later I was feeling much more chipper.
I saw an immunologist when back in the UK. Had the initial batch of skin prick tests which showed only a reaction to house dust mites, although houses, dusty or otherwise don't raise so much as a sniffle. Apparently there is a crossover between this and a shrimp protein, so I had some blood tests done which showed a reaction to the shrimp protein, crab, lobster and oyster.
On the night in question I had eaten a quantity of all of the above, which probably explains the severe reaction and why I had previously been eating shellfish with no ill effect.
The upshot is that it is narrowed down to being shellfish that I might react to and the Epipen is a precaution. Aside from flying I often travel alone and in countries where I might have trouble communicating. Whilst it is relatively easy to avoid consuming shellfish - should something happen I like to think that I'd have the ability to sort myself out sufficiently to figure out how to get some medical help.
Incidentally I still eat in fish and sushi restaurants, just obviously not the shellfish, where there is a chance of cross contamination and have suffered no ill effects since.
Hopefully I'll get some AME clarity later today.