Hi all, thanks again for posting and here's the update:
GP looked at it and said whilst it is not a 'classical' bullseye rash the truth is that in somewhere between 30%-50% of cases there is either no rash/a diffuse rash/a.n.other presentation and that she thinks it could be Lyme, though equally she knows it might not be Lyme.
Spelt out 2 options: blood test but that not actually reliable as can present a false negative (IE yay, you're fine but only to discover later that you're not fine and now left with something much more dangerous and much harder to treat) or straight to antibiotics.
Her actual words were: "It's 50/50, I suspect it could be Lyme but there is a chance it isn't" 
She offered me either the blood test and then review when result back (bearing in mind it could come back as negative but that actually not being reliable) OR start a 2 week course of Doxycycline now. She then said if it was her, she would take the antibiotics given the potential enormity of not treating. I asked my pharmacist (who I know reasonably well) what he would do and he said the same as Doc; namely he'd take the antibiotics.
If I was perfectly healthy right now, based on the above I'd just take the antibiotics, however reality is that I'm coming at this from a place of contemporaneous pretty shit health. Have savage gut issues stemming from combo of Gastritis and, separately, long bout of food poisoning so a pretty relentless period of vomiting followed by 24/7 nausea which remains (have lost a stone in the past month to put it into context). Separately to that, have also needed to take Prednisilone (steroid) for past 3 months so immune system on floor already. IE I'm immunocompromised, weak as fuck right now, uber concerned any worsening of gut issues will take out the one tiny meal a day I've been managing late in eve (IE at end of day and only after multiple doses of 3 chemo strength antimetics) and what I'll do if I literally cannot then eat at all?
For the same reasons as anyone with half a brain, I don't ever take antibiotics lightly (think maybe once in past decade?) and that - coupled with the hideous looking list of side-effects (esp. the gut ones given I'm pretty weak from ongoing nausea already) - is what's making me think twice before I commence the course.
The one thing that strikes me though is that I know without a shadow of doubt that if OP + update was written by someone else, I would be posting 'sod the fact the Doctor's not sure, she was clear is a fair chance it IS Lyme/so sod the side effects/take the fucking meds already/you're insanely lucky to have this chance given so may people do not and end up with pretty savage long-term consequences' etc etc.
I have maybe another 2 days max to decide take them or not but after that it's shit or bust; either I don't/never had Lyme and I'll be just fine OR it is Lyme and I'll have missed the window of efficacy for knocking it on the head. So I do need to make a decision asap. That's where it's at right now.
If you got this far, thanks for reading 