I know how you like this, so I'll try to be as descriptive as possible for you.
He has consented to one photo, and it's not very spectacular, but I shall post it after this from my phone.
DH has a sebaceous cyst, centre back, been there for years. he had it cut out about 5 years ago but GP clearly didn't get the sac and it started to refill. It got back to the size it always was (walnut?) and was pretty static, not giving him any trouble or getting bigger.
A week ago he said 'something's changing' and asked me to have a look. So I said 'well, if you insist' 
- it was suddenly getting a lot bigger and redder and looked ready to explode. He went to the nurse who scheduled a GP incision appt but they did say it could be 6 weeks. She was determined it was not infected so no antibiotics. She did have a 'gentle prod' he said.
Monday we'd been in the car all day, hot, with his back against the seat. He felt something was wrong; 'wet' and again asked me to check. His tshirt was covered in muck and this thing was obviously draining, so he stood in better light by the back door - luckily I was standing slightly to one side, wiped it over (with no pressure at all) with a sterile swab and it literally exploded - what hit the back door (audibly!) was a trail of revolting contents. The walnut lump now seems to have gone from hard contents to liquid contents - like it's liquefied. I cleaned and covered but it was self draining, like a constant stream of the stuff. Soaked through dressings.
He's been to the GP who has put him on a long course of antibioics and told him to get me to clean and re-dress it twice a day. I can't understand how so much gunk can be inside the thing - so much has come out - everytime I replace the dressing I use about 6 swabs. It literally runs down his back. loads comes out, it looks loads flatter, then the next time I change it it's back up raised again and more comes out. I am a sporner, and it makes even me a bit bleugh.
Wondering when the antibiotics will kick in.
Photo to follow.