will a chip approx 5mm across it’s widest point mean an MOT fail or just an “advisory” to get it fixed?

phone autoglass and you’ll not need to worry.
I think the general rule is if it’s smaller than a 5p then you avoid be ok.

think it depends on whether its in the viewable screen area i.e. within the wipers sweep?

some advice here

As above depends where it is more than anything.

Half way up on passenger side

Should be fine in that location, but small chips can often be ‘repaired’ on your insurance free of charge without screen removal – have had 2 on my screen done this way.

So is it a fail because it “restrict” visability ?

Beacause if that the case….

What about all those pillocks you see driving about with a sat nav stuck right in the middle of their windscreens ??

If you leave your sat nav (or just mount) on in the ‘viewable’ area in an MOT then it would be a fail to (I forgot to remove mine once but fortunately the garage had the sense to ring me to advise and remove it themselves…). Not actually sure where the law stands though (i.e. could police stop you for a sat nav positioned in certain areas on the windscreen)? Dunno about being a pillock, I don’t think where mine is positioned obstructs my view (inc. of small children/animals…) when driving.