betterfromabove.... you need to visit an ATC radar unit and see what goes on!
<<A PA28 cleared to route BUR-Ascot not above 1000'QNH cannot be meeting those 5nm/5000' criteria surely? Or do these criteria only apply to unknown traffic? So what would apply in this case of the PA28?>>
Why should it need to? All SVFR traffic will be identified and ATC will provide standard separation (1000ft vertical or 3nm horizontal) between it and IFR or other SVFR traffic.
During my time at Heathrow I must have seen hundreds of zone infringers (curse them!). We did our best to maintain horizontal separation and provide information to other pilots. If they saw the infringer they could provide their own separation, which could be considerably less than that provided by ATC.