I've glued it back together with Araldite, and await the response from the retailer (UK BIKE STORE LTD).
For a light that cost almost eighty quid, I'm not impressed.
25 Jan Update: The UKBikestore has got back to me, saying this:
We don't have any spare mounts here but I've just spoken to Extra UK who are the UK Topeak distributors. They have agreed to send one out to us which should be here in a day or so and I will get it straight in the post to you.Hopefully the replacement will be here soon!