Can anyone explain the relative merits of heating the heatbank directly or
indirectly?

Direct enables the full boiler output to be used. Also, you can have as many
direct heat sources and sinks (i.e. radiators) running off the heat bank,
like a huge Dunsley Neutraliser.

Indirect has the advantage that the controls are exactly the same as a
conventional system. It also allows sealed pressurised primary circuits.
This is good as it allows you a full choice of modern condensing boilers
(some of which can't run vented) and has all the benefits that go with
sealed operation (see the FAQ).

You can mix and match if desired. This would involve having a modern
condensing boiler on the indirect coil, whilst having your Aga, solar
heating and/or solid fuel boiler off the heat bank.

Radiators can be run off the heat bank (essential if you have even a
partially direct system) or off the boiler primary circuit, leaving the heat
bank solely for hot water heating.

A while ago someone recommended the Worcester Greenstar(?) boiler, any
more
good/bad reports?

I've had it almost a year now. Runs like a dream.

Christian.