Squier Vintage Modified Strat pickup change and set up

Here I change the pickups on Peter's Vintage Modified Strat and give it a set up. All the usual things here; 1st fret action, last fret action, neck relief etc. On this set up I noticed there's an annoying grounding buzz on high gain on my shed amp... I've since noticed it on finishing a couple of other single coil set ups - and each time got me worried that I'd done something wrong :-) But I need to replace that amp!