@Dom Williams, Firstly, Love the filename that comes up when you open the CMM file. Glad it isn't just me that gets grief when I get 'homers' done at work! 😆
Secondly... From experience, the corrosion will have put pits in the surface. This will just give the fluid somewhere to wipe past the seal, having a more negative effect than the bore simply being oversize. Typically a hydraulic sealing bore of that size (0.7"/17.78mm) will have a maximum tolerance of 50microns ish, so yours seems on the upper end but not outlandish. You could try getting it honed out to clear up the corrosion and see how that ends up?
There is a company called Hone-All in Leighton Buzzard that are good - may be worth a shout.