Problem identified, I think.
The rims are 13.4mm wide. Schwalbe recommend 25c tyres for rim widths down to 13mm, but Marathon Pluses are a special case, with their thick construction, and this is pushing it. It can probably be done eventually, but...sod it...Fortezza TriComps are virtually puncture-proof, half the weight, more grippy and a hell of a lot faster.
Oh, and campagman, it's a really small tube.