If I were in your situation (and I really wish I was ) I would go and see the amp and try and lift it. If you think it is managable, snap it up for the £300. If you then find its too much (weight or power wise), you have 2 options...
1) Hang on to it and use your 410 for gigs and keep the fridge as a 'fun time' amp in your rehearsel space (or room / whatever)
2) Sell it on... You could probably make a profit on it and the case
I've only ever played through an 810 once and that was the ABM 810 with an ABM1000 sat on top. I am a 5'7/5'8, fairly weedy 18 year old and this thing was roughly the same size as me. I tried lifting it, and I found that I could 'manage' it, but I wouldn't want to do it by myslef everynight. However, the sound was just incredible. I cranked it up a little and was literally stepping into the sound of this thing. I'm hoping that 2 of the ABM 410s will have a similiar effect without the manoeuvrability issues