Thanks you so much for your very detailed response. I really do appreciate your comments and they help me to understand the dynamics possibly going on here.
I feel I have misled you a little on one of the points though...my 6 extraction that took place a fortnight ago does not have an implant in it. It is still healing.
The 5, where I have the new implant, only lasted about a year (15 years ago) with the root filling in it before I cracked the tooth and it had to come out (admittedly after developing the periodontal abscess I spoke about). I can pm you my X-rays of the site prior to implant if that's helpful. I do not have any post treatment.
I had x-rays, but no CT scan. They are not routinely available here and I did ask about being referred for one prior to the implant but was advised that in all his years in hospitals working with ct scans for implants, he felt they didn't give any real advantage over several well placed apical x-rays and gave far more radiation. That kind of thing that subjects patients to radiation has to be justified here in the UK.
Emailed my dentist today to give him an update and he is confident that he did not penetrate the sinus membrane even if he did actually penetrate the bony sinus shell. He also said that he could see the shell of the sinus and it was intact on the tooth he extracted.
My antibiotics course is finished now. The discharge is far less this afternoon than it was. Not sure where to go from here.