I trolled through the forums and found some info on this, but there was still nothing clear cut saying that the genuine towbar could be fitted with the very popular Berklee 380 muffler, because of the muffler's size increase over the stocker.
I defnitely did not want to go back to my stock genuine muffler (which is practically brand new, and has been sitting in my garage for 5 years) for so many reasons.

Anyway, I bit the bullet and went to Allin Towbars on Richmond Road (who actually supplied the 380 genuine towbars to the factory). About three different installers came out and crawled under the car, mumbling under their breath to each other and scratching their heads. They thought it might be a problem, but were fairly sure it would only be minor and an exhaust shop could tweak the brackets and rubbers on the muffler to get it all to fit. They said they might have to remove the muffler to fit the towbar, and then stick it in the boot for the exhaust shop to reinstall.

Well, long story short, today I picked up the car. They managed to fit the genuine heavy duty towbar with the Berklee exhaust. I think they said they had to slightly modify one of the muffler brackets and fit a couple of shorter rubbers, but they were fairly confident I wasn't going to get any rattles (and if I did, there was still plenty of room for an exhaust shop to get it to fit).

So I happily cancelled my exhaust shop appointment. Allin Towbars didn't even charge me any extra for the extra labour of moving the exhaust around and making a couple of small mods to get it to fit - so thumbs up for them.

So anyway, there's your definitive answer. The genuine towbar can definitely be installed with the Berklee BS0655 which seems so popular on this forum.