I see from their website that the Liberal Party is backing a Yes vote in the referendum on AV, while stating that it isn't the best system and reaffirming their party's support for the Single Transferable Vote in multi-member constituencies:

Welcome to The Liberal Party website

Support for Yes to Alternative Vote Campaign

The Following Motion was passed at the February Meeting of the Party’s National Executive Committee:

1. The Liberal Party NEC recognises that Liberal Party policy calls for a transition to an electoral system based on a Single Transferable Vote in multi-member constituencies at the earliest opportunity. The NEC remains committed to the campaign for, and the promotion of, such an electoral system.

2. The NEC recognises that the proposed 'alternative vote' electoral system is not a system of proportional representation, nor it is a satisfactory electoral system.

3. With a degree of reluctance the NEC resolves to align the Liberal Party with the 'yes' campaign in the forthcoming referendum on the adoption of the alternative vote system. It does so for the following reasons:

i The AV system is the only reform proposed in the referendum. It is recognised that even in the event of a vote the reform will be minimal and not wholly satisfactory, however, the reform is, on balance, favourable to retaining the status quo.

ii The NEC sees participation in the 'yes' campaign as the most effective way to promote the party's existing policy.

iii The NEC sees the adoption of the alternative vote system as a potential 'stepping stone' to further reform, in particular the adoption of STV in multi member constituencies.

4. The NEC affirms that nothing in the above does, or is intended to, change the existing policy of the Liberal Party or in any way dilute the party's commitment to STV.