This started off as a reply to all the comments on the last post, then I came to the conclusion it would merit a post in its own right.
First of all, thanks for all the comments and good wishes, everyone.
I've no happy "poultice full of snot" story to share yet. The pink spot on the back of his heel is still there but seems less soft. There is a small amount of black stuff coming out where the poultice meets the top of the PHW wrap - I'm not sure if this is a little bit of thrushy goop coming from his frog or if it's old black pus working its way out from the bottom.
Right now, I'm still tempted to rip off the hoof wrap, unplug the hole and really go after it for a couple of days. I'm reluctant to head down the antibiotic route for two reasons, the first being that I've never found them to help with hoof abscesses before. Years ago, we had a similar protracted abscess story with our little pony Rosie which the Vet initially treated with penicillin, after opening the abscess. Ten days later, he was back and ended up removing literally half her sole to try to get it to drain again. Apparently antibiotics kill the infection, but the pus and dead bacterial matter is still in the foot, only now it's no longer liquid so it's ten times harder to get it to drain.
The other thing which is making me think twice about antibiotics is Aero's history. We spent the entire summer of his seventh year treating an infection in his right foreleg - we think he picked up a thorn during his first ever event. He won the event, but lost the rest of the season while we went through a seemingly endless cycle of poulticing, medicating, resting, bringing slowly back into work, leg swelling up again. He had three courses of different antibiotics and in the end the Vets and I all agreed they weren't helping and the thorn was going to have to come out by itself. What eventually got it out was a garlic poultice, but that was very severe and actually burned him like a blistering agent (some of you might also be old enough to remember blistering!) and I'm really reluctant to do that again.
I'm going to keep up the soaking and poulticing for the next day or two, but on Thursday I will assess where we are and I may revisit the antibiotics idea. I'm hoping to turn him out in the arena for a while each day (movement would help), but we've just had yet another night of torrential rain and unless it clears up really well this afternoon so that the arena dries out, he's going to have to stay in all day.
The other thing I'm trying to do is to get my hands on a bottle of a homeopathic treatment "Hydropathics Chronic Abscess." It's worth a try, especially since he has a history of his own immune system not working too well.
I'm off to boil the kettle and soak the Animalintex now...