That looks like Stictoleptura cordigera. It's not currently listed as a British species, although it's present on the continent. We do get occasional accidental imports of foreign longhorns in timber, but this might have hopped the channel and be new to Britain if it got established. Someone needs to get down there and get a specimen.
I collected one yesterday but later released it. Mark Telfer says it's a first for Britain and that he wouldn't collect one himself, so I'm unsure whether to go back today and get one. I suppose I will. I'm collecting Litophasia there for Chris Raper at the moment.
Does it have to be collected or observed by someone other than myself to be confirmed, etc, I'm not sure what the rules are. I'm at the Angela Marmont Centre tomorrow (Friday) to use the photo-stacker so I'll enquire there as well