Sounds like 6mm, with 0,75 mm pitch (= 33.87 tpi). This is a metric fine pitch series, used by model engineers among others, and available in the UK, replacing the old 32 tpi series. A specialist tool supplier (not your local DIY store) could get you a die for it. But if you're only going to use it once in a lifetime, just buy a couple of those screws - that will probably be cheaper.
:wave:
Patrick