I notice a difference visually on pre 99 models

to 99 onwards when looking in the engine bay. On the earlier models, the oil filler is on the right it seems, where as I seem to recall its on the left when looking at later models. What's the difference in the engines if so.
I can see the obvious difference on the big square block v the alloy looking smoother inlet manifold ( i believe it is) but any other significant differnces in the engines?

No, the oil filter has always been on the nearside, the housing changed from 'small nut' to 'big nut' but that was very early on. Also the inlet manifold changed from metal to plastic a little bit later but before the XA model year.

As far as I know the ECU changed internally, but quoted output figures remained the same, though couldn't tell you any difference in driving.