Thanks Archangel, macmackit,

Based on your suggestions I examined the include file INT-TIM0.PBP

The "ROLLOVER" was the DEFINED label for the Interrupt handler

The manual says this is case sensitive, and when I capitalised the label ROLLOVER at the start of the asm routine it all worked

Seems that in V2.5 the label does not need to be capitalised, whereas in V3 it does :-)

Many thanks for the suggestions

Angus Anderson