I love my neutered boars - all three of them! (Not to mention the ones that have sadly passed away.) x)
Boars don't interfere with the sow hierarchy itself; they just slot in where they fit in around the existing hierarchy as they need to get on with all girls in order to be allowed to mate. By introducing a "husboar" to a group of sows you are working very much along piggy instincts. Sows live in a core group with one male (who has to be accepted by them); group dynamics are wired for that kind of scenario.
Some boars are born leaders like my Hywel, who is the unquestioned president of the Tribe (and who I specifically chose for the job of following in the large pawsteps of a very competent patriarch, founder husboar Llewelyn).
Other boars are more laid back and are letting the sows dominate, but are using their considerable skills to keep the peace between several strong personalities, like Terfel, who has turned out to be a master diplomat and worth his weight in gold in successfully integrating a very fear-aggressive elderly half blind sow into his group of formidable cataract ladies; Mali is now one happy, relaxed girl and very much one of the group!
You will find that most people who have introduced a suitable "husboar" to their existing girl group have had a very positive experience. The main sticking point is initial acceptance, especially by the top sow. Ideally, it should happen quite quickly once they start interacting. If you can, take all your girls for a trip out, especially your top lady, for a round of boar dating at a rescue to make sure that they click, then you are onto a winner! Some very dominant, mostly older sows are not too keen on getting a rival for attention; that is why I always recommend dating at a good rescue with these things. The chemistry needs to be right! In your case, I would recommend to look out for a rather laid back non-dominant boy who will not overturn the apple cart.
As far as I know there are one or two rescues around Bristol that have a boar neutering policy. Not sure whether Glynneath GP Rescue has any (Suzy prefers to boar date and is very good at it). Dating at a rescue has the advantage that you only come home with a husboar if there has been success.
https://maps.google.co.uk/maps/ms?msid=209994852075231951564.0004b8fd9391b4257d8eb&msa=0
Of course, what you cannot control are group dynamics over a longer space of time when you have changes at the top and the group needs to rearrange itself in a new hierarchy. But that goes for any kind of piggy group that you have for several years, as piggies get older/die and new ones come in, and the whole dynamics shift accordingly.