What do people think about using a variable speed pump and temperature
sensor instead of a TMV? Would this approach would also give better
stratification inside the thermal store? How easy is it to find a
variable/multi-speed circulator pump?

I think most pumps use squirrel cage rotors, so a variable frequency
drive (vfd) could be used to give variable speed on a standard pump:

http://en.wikipedia.org/wiki/Variable-frequency_drive

These aren't cheap, but the upside is they have lots of control inputs
- so sensor hook-up would probably be straightforward.

A 150W VFD would probably be sufficient (this isn't the right one, but
it gives you the idea):

http://cgi.ebay.co.uk/HP-SINGLE-TO-T...QQcmdZViewItem

You'd need both a flow switch and a temperature sensor. Probably just
hook the flow switch to an interlock on the vfd - and an appropriate
temperature sensor (that has a linear voltage output) to the external
speed adjustment on the vfd (which can usually accept an external pot
or something like an external 0-10Vlevel).

Finally you would need to achieve the appropriate control action -
with the right choice of VFD it may be possible to do that in the
firmware. In my experience of using a VFD to run a 3 phase bandsaw, I
found my Omron unit had everything I could possibly need and lots more
besides. They're designed to be ultra-flexible for a huge spectrum of
uses.

Perhaps someone else would like to comment on setting an appropriate
control action - vfd's usually offer PID control:

http://en.wikipedia.org/wiki/PID_controller

You'll probably find there's also lots of manufacturer info about
hooking up temp sensors to vfd's for industrial processes.