Just a quick summary of replies and thanks for all those suggestions!
I forgot to mention that the PIC is a 'C84 but everyone was aware of
this...

There were lots of replies indicating that I should use a Mosfet or
relay instead of a transistor to switch in Vpp. The Mosfet is good as
it has very little voltage drop but a relay is a bit messy really.

Also suggestions that I use a DC-DC converter which is a good idea and
one that dawned on me just as I posted...Aaron Sliwinski suggested
looking on network cards or using MAX232s as cheap converters which
is really interesting!

I was originally thinking about using the ISA 12V rail but Mike Watson
said that "From experience, the PCs 12V rail is not very useful. The
PC to PC variation is too high." So the best thing to do is use
a switcher IC like those used for programming flash memory - at
least here you _know_ that you're getting enough voltage for Vpp.

Also Andy Errington said:

"It is important to note that the Vpp voltage is not actually
used to power the programming process, it is simply the presence of a
voltage at this level that sets the '84 into program mode.  The actual
programming voltage for the EEPROM is internally generated."

This I wasn't aware of and I was assuming that the process was similar
to EPROM. It means that a) I don't have to supply much power for Vpp and
b) that I should go back and read the data sheet more closely :-)
You can get switcher ICs capable of a couple of milliamps for a pound
or two.

Thanks again - this seems to be a really active mailing list. 70 messages
this morning!

Conor O'Rourke
Dublin, Ireland