Is it best to put the timeswitch between the consumer unit and
the isolator switch or should it go between the isolator switch
and immersion heater?

Personally, I'd replace the existing switch with it. If you do this, you
must choose an immersion timer with built in DP switch with 3mm contact gap.

i.e.
http://www.tlc-direct.co.uk/Products/SMFST17.html

Christian.

P.S. With a modern well insulated cylinder, the timer is unlikely to save
much money, although if the immersion is noisy, you might get more sleep.