Update...
I rode my bike around my factory (polished concrete floor), I couldn't lock up the rear no matter how hard I jumped on the lever. I knew it wasn't right. Took the caliper off, 1x seized piston. The outside pad was actually bent. Crazy.
Cleaned and rebuilt the caliper (quite easy actually), fitted new EBC pads and hey presto, a working rear brake.
Also, I fitted Pirelli Angel ST tyres, I'm still easing them in but so far impressed. I did want the Michelin R 5s but these were half the cost. Hopefully not a "buy cheap buy twice" situation...
Looking forward to a full week of commuting ahead.