Halfords was also just past the closest train station, when you posted I was wandering through Charlton village. I got to the station about 45 minutes before I would be allowed to use it so I carried on to Halfords to buy spares and fix it there.
1st problem: Halfords don't sell inner tubes thick enough for a 45c tyre so I need a new tyre as well.
1st good thing: they have 1 Marathon Plus left and it's the right size.
2nd problem: they don't sell 700 inner tubes with schraeder valves.
3rd problem: I don't have a presta pump.
4th problem: The rim tape is a poor fit and has exposed the spoke holes, this is a puncture waiting to happen.
5th problem: Halfords are out of stock.
So I now have a presta valve fitted in a schraeder hole, I don't know how long it will be before that goes pop. I have a front tyre thicker than the rear, a little odd but it rides fine. I have a terrible bike pump that can't manage a decent pressure. I have a pretty blue colour inside the mudguard where the slime sprayed out and covered all in it's path and I have a legendary tyre which I'm confident I'll puncture soon.
Now to order new inner tubes and rim tape before it happens again.