Sorry for the late reply, but
abs imo doesnt work of wheel size
Well, yes it does (sort of), just not in the way that the garage thinks. There is a toothed wheel and the electronics picks up a wheel speed signal from that. You can define max d-theta-by-dt (sorry, its late and I can't be arsed finding the Greek characters) that would be expected if the tyres are not loosing grip. For a variety of reasons, the lowest speed that the system works down to is a function of the tyre size...but it isn't very critical (you'd like it to work down to 0 mph, and it can't, so anything else is a slightly unsatisfactory compromise) and your tyre rr isn't much different from the original.
just how hard you plant your foot on the brakes
In a plain ABS system, brake pressure isn't a factor, just whether the tyres are gripping. It might be if you have brake assist, or something, but if is just ABS, no.
Remember that the brake pressure that would be fine on a dry road isn't going to be all that fine on ice or snow or even wet....and even how grippy the tyres are is a factor that the electronics knows nothing about.
and traction control just stops any wheel spin.
Yes, and it works in a vaguely similar way to ABS, but only the front two wheels being controlled (backing off the engine, and applying the brakes depending on the system). So, it depends on rate of change of angular velocity.
Where the garage would have a point is if you fitted even slightly different sized wheels front and back - there is enough tolerance in a typical system to allow front and rears to be differently worn, but not much more than that - but you've fitted the same size at both ends, so that doesn't arise.
it works out at a 2.1% difference..
I've just run this past willtheyfit, and that gives a 5.82% difference. Still doesn't make a difference to the point that they are the same size at both ends, though...
well of now invalidated parts of my warranty by swapping to smaller wheels but also that i've possibly caused lots of problems now and that i should have the car reprogrammed with the wheels set at this new diameter and at a cost of around £50 !!!!!!
Well, I'd guess that they'd tell you if you didn't use Ford fuel in your tank and Ford air to blow the tyres up with, you'd invalidate your warranty, if they thought that they could get away with it. Unless there is a ground clearance issue, or the effective offset is wrong, I can't see anything other than the dealer being full of stuff (spelled differently, but still stuff and brown) and trying to cover their rear. If you look in the manual and check the various sizes of tyre fitted to other models of the same car you might even find something as 'extreme' as these fitted to another model.
Watch out for the speedo error, though. You might want to check on a sat nav, at constant speed, how it seems to be working out in reality.