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Thanks sqad, the cut was maybe 1/8th of an inch deep maximum, and no earth had found its way visibly from the back of my hand. But as I did my best to close it asap for obvious reasons I wondered it that cuts the oxygen off.
Anyhow, for those needing to know in future I spent yesterday doing the research via the doctor and firstly although they'd always called me in the past said I was due for a booster as it was 16 years. I thought I'd had 5 already but booked one, but went to the walk in as didn't want to wait till Monday and wanted my finger seen before then.
The rules are you can have a booster up to 72 hours after an injury should you need one, and the reason my doctor thought I wasn't fully insured was the hospitals don't tell them when they give you one (as I must have done after some barbed wire) so wasn't on my records. They said I had to tell them (they didn't at the time). Anyway, they were pleased with the progress, impressed with my use of butterflies (as the doctor used one the last time it happened) and didn't just give me the tetanus booster (the last ever) but a DTP with diphtheria and polio although I had one as a baby.
Not sure the significance of that nowadays but they also confirmed the injury wasn't susceptible and healed nicely so at least I've learnt a lot and didn't have to cancel my arrangements for Monday in exchange for a 90 minute wait, and no magazines so absolutely nothing besides the crowd of patients tapping on their smartphones with the occasional one ringing (including my old type twice) to break the monotony, but got it sorted in one go.