wondering if the standard rocker covers will fit after I've fitted a set Yella Terra roller rockers, or do I need a taller pair, and will a set of a carby windsor fit.

Yenerally the bolt on non-adjustable rockers will fit under stock covers. Most of the carby rocker covers are actually talller so should fit ok too, but if you are fitting carby rocker covers on an EFI engine you need to somehow route the pcv system properly.

cool, I'll be going a set of stealth terras, so is there an easy way to tell which is EFI and which aren't, I'm looknig at getting a set of Ebay to dress things up a bit, failing that a respray of hte stock ones is on the cards

Just paint the ones you currently have whilst you're installing the rockers (ST2027). Unless of course you want chrome.

If (and I doubt they will) your current covers foul with the rockers, then you can get taller cork gaskets.

The EFI covers will have the oil filler tube like your stock covers with the vent tube fitting. The ones on Ebay are generally to suit carby, EFI ones are rare. You can fit them but you need to rig up a way to route the rocker cover gasses back into the intake liek the EFI ones (the hose from the oil filler to the throttle body). Just venting to atmosphere or blocking them up will cause problems.