It sounds like you hav your answer. If the guard, and by guard I think you mean your hand postures while in stand up, are what you need to work on, then a decent boxing coach will help you quite nicely.
I would pipe in that the people I have worked with in Kyokushin did indeed have a somewhat weak guard against HAND technique to the head, as the international knockdown I experienced had no hand blows to the head as part of the rules set.
Either way, Kyokushin and Judo would compliment each other nicely. My Shifu got his start in Judo, as a child. He never ceases to praise it as a good and decent art.
So I actually trained there today, a two hour beginners sessions with an hour on striking and an hour grappling. Aside from various Wing Chun memorabilia on the walls, it seemed to be primarily a MT/BJJ place.
Standup coaching was of a good standard, although the coach did tell me to keep my foot flat on the floor for my roundhouse, whereas I thought I was supposed to bounce on the ball of the foot instead. A more experienced poster could probably explain which is best. Other than that it was mostly combinations on thai/focus pads followed by counters to crosses and jabs, which the guy I was with seemed pretty proficient at. He told me he had trained Wing Chun for three years and MMA for one. No sparring, but it was the beginners class.
Grappling was interesting however. First we went over some basic transitions; escaping side control to guard, bridging etc. The instruction in this seemed pretty good, although I don't have a grappling background enough to comment on it. What was really interesting was when I rolled with the guys at the session. Bear in mind that aside from a few ground fighting lessons from a JJJ instructor and 6 months of Judo when I was ten, I have no groundfighting background. However I was still able to tap the other guy about 80% of the time, fighting against three different people. Okay two of them were skinnier than me and it was a beginners class, but I was surprised that I was able to win so easily. I got the impression that with a few weeks of decent coaching, I could dominate the class. Like I said though, most of the instruction seemed pretty good. Maybe they don't roll enough in class?
As an aside, the one time he DID mention Wing Chun was when talking about defending from strikes when your opponent has the mount. He said something about keeping your opponents hands away from you as 'Chi Sao' and did a few moves that I recognised as sticky hands. Aside from that, the class was strictly MT and BJJ.
Also, in regards to the grappling coach (the judo guy). One of the students informed me that he was the under 18 British champion (or something along those lines) which may explain why he can't be found online in any adult competitions. However he's traveling at the moment, so I wasn't able to see how good he was.
To be honest, although the gym isn't a bad place, and I could probably haggle with the owner in regards to the membership fees, it's just too far away to merit going there twice a week. It's about thirty five minutes from where I live, and what with petrol costs and general impracticality, I think I'll be sticking to the clubs local to me.