Right, after a bit of digging I found it!
The Green Book - Parliamentary Salaries, Allowances and Pensions (pdf)

The pages on the Additional Costs Allowance are pretty vague (I suppose they don't want to shoot themselves in the foot too much), but:

"3.11.1. Definitions

Main home

When you enter Parliament we will ask you to give the address of your main UK home on form ACA1 for the purposes of ACA and travel entitlements.

Members are expected to locate their main homes in the UK. It is your responsibility to tell us if your main home changes. This will remain your main home unless you tell us otherwise. The location of your main home will normally be a matter of fact. If you have more than one home, your main home will normally be the one where you spend more nights than any other. If there is any doubt about which is your main home, please consult the Department of Finance and Administration."

The following is also quite amusing:

"3.3.1. Principles

You must ensure that arrangements for your ACA claims are above reproach and that there can be no grounds for a suggestion of misuse of public money. Members should bear in mind the need to obtain value for money from accommodation, goods or services funded from the allowances.

3.3.2.
You must avoid any arrangement which may give rise to an accusation that you are, or someone close to you is, obtaining an immediate benefit or subsidy from public funds or that public money is being diverted for the benefit of a political organisation."

Burn the witch, I say!
(Son of Sam-i-am We're all as mad as hatters here, Tue 24 Feb 2009, 12:17, Reply)