I generally buy from them (as supposedly does Brian May/Radiohead/Iron Maiden/Eric Clapton)! They have a big custom test rig and seem to test the hell out of their valves generally. I seem to have been very unlucky with original valves in my Herbert over the years from new. I even sent my amp to Germany, back to Peter once, who couldn't find anything wrong with it, just retubed it. Wasn't that long before they seemed to blow again. Watford's are still tubes so they won't last forever or be indestructible, but I've had a LOT less problems with their valves in my Herbie than other makes such as the TAD and Rubys that were originally fitted or fitted as a replacement when I had problems. I haven't changed thm for a good while now.
So I can't necessarily vouch for how good these will sound in your Diezel. All I can say is it might well make them more reliable! In general WV seem pretty well respected and have done kits for Marshalls and Mesas for years. They never used to have any Diezel kits (this was news to me) and when I phoned a few years back now and spoke to Derek there he seemed pretty knowledgeable. He didn't have any suggestions for what would sound good in my Diezel though at the time as he hadn't tried one. That was a while back now though so whether he's just got sets of tubes that fit and sound nice in other amps, so is selling them as a kit, or if he's since been borrowing Diezels and experimenting with them who knows!?!
I'd be interested to hear them though. Currently my Herbie has a mixture of their Harma STR and Retros in (Watford's own-brand). These EL34s have lasted a LOT longer than any of the other sets I've had in it; they're yet to blow after an annoyingly short time at least! Am I happy with my Watford Valves? Yep! Certainly lasted longer at least. These aren't their own brand though. These are JJ/Tesla or Electro Harmonix, but WV have had them cryogenically treated. So I have less of an idea how they'd sound than my WV own-brand non-cryo ones. Whether the cryo valves sound better or not remains to be seen. Haven't seen any reviews of these directly, but I have seen reviews of their cryo-treated own brand to give you an idea. WV have a link to the reviews in Guitarist (probably the biggest selling UK guitar mag, not a tin-pot internet site!), and guitar buyer (another proper mag). In the Marshall at least the cryo results seemed to make a difference, but not necessarily for the best, depending on the tube. At least they seemed to think all of them were a good improvement over stock tubes in the Marshall at least. Guitarist:
http://www.watfordvalves.com/pdfs/git276.rev_valves.pdf Guitar Buyer:
http://www.watfordvalves.com/pdfs/guita ... rd_4mt.pdf
If anyone has money kicking about for a set then I'd be very tempted to give them a try. I'm trying to save for a 212FC at the mo though so don't want to be unnecessarily spending money on tubes when mine all seem to work! I've got "notify me when a reply is posted" though, so hopefully by the time I next do need a set someone will have posted a verdict here!

Otherwise phone up, ask to speak to Derek, and ask him why he thinks they are better!