Was in past my local Phones4U Store when I saw they had the Sony Ericsson X10 Mini Pro Handset reduced in price (last time I saw it, it was going for £130+) - it's basically going for £99.95p as a PAYG upgrade to your handset or an additional £10.00p if your purchasing with a Vodafone/T-Mobile SIM (O2 is an extra £10.00p for the handset).
Thought it prudent to post in case anybody's interested in one - note that it's also available online but you need to purchase it with a PAYG SIM from what I was advised by staff in the store.
I'll add in some additional links to tech specs and custom ROMs within the first post.
Edit - Here's the price breakdowns for the PAYG options, if your purchasing as a non-handset upgrade.
T-Mobile - £99.95 Plus £10 T-Mobile Airtime - Total: £109.95p
Vodafone - £99.95 Plus £10 Vodafone airtime - Total: £109.95p
O2 - £109.95 Plus £10 O2 Airtime - Total: £119.95p