Ok, so in the light of your point about taking longer in the interrupt routine, I went back to basics...

Code:
ADC_Sample:
@ INT_DISABLE  TMR2_INT     ; enable Timer 0 interrupts
    toggle PortC.3
@ INT_ENABLE  TMR2_INT     ; enable Timer 0 interrupts
@ INT_RETURN
This gives me an interrupt frequency of 11.44khz (ie 5.72khz on the scope)...so my interrupt routine is severely skewing my interrupt! (told you I sucked!)

It's late here in the UK...need to sleep on this one - many thanks for your help.