If you phone Vodafone sales on 08080 044 423 and ask about the HTC Desire, you should be able to get the HTC Legend on 24 month contract for £16.25 per month, down from the usual £25. This includes 300 minutes, unlimited texts and 500mb internet.
I phoned to haggle for a new contract with either the HTC Legend or Desire. I was told there were no Desires in stock at the moment and they didn't know when new ones were coming in, so they were authorised to offer the Legend at £15 per month for the first 21 months, then it would go back up to the standard £25 per month for the last 3. This averages out at £16.25 per month over the two years. I didn't have to haggle at all - this was offered to me as soon as I mentioned the Desire so I think it's a fairly standard deal at the moment. Also I'm a new customer, so this wasn't leveraged by previous loyalty.
Presumably this will only last while they're out of the Desire, so get in quick and make sure you say you want the Desire!
As a comparison by the way, getting the Legend on vodafone online through quidco averages at £21.25 per month.