After lots of cross-referencing part numbers/suppliers, it would appear this pressure unit is also used on some late 70's & 80's BMWs, notably 6 series.
Research into these cars & parts indicates the o-ring on the brake pressure accumulator is 15.3x2.4mm.
Ordered a pack of 2 for the grand sum of £4.06
Will see how they fit & post result for future reference.