Update - good news! So after a day of faffing and failing to get a bolt of any size to fit snugly, I rang Rob at Ratrace for a chat (he serviced the car in April). We both agreed that driving the car with a loose / absent radiator screw was a bad idea… so he suggested removing the radiator and bringing it over. Smart man. It had not occurred to me that (obviously) the radiator was easy to remove… so I began draining the fluid – and at Rob's suggestion – collected it in a clean container to be filtered and reused at a later date. With the radiator bagged up, I headed to Ratrace for assessment. Rob was able to tap out the radiator thread to 7/16ths and fit a new bolt along with a bonded seal washer (Dowty washer) – so more education for me. Back home, I was able to reattach the radiator, and then refill / bleed with fresh coolant. I ran the car up to temperature in the garage, let the fan cycle three times and shut it off. Using a large cardboard box (flat) as a tell-tale drip tray I left the car over night. No leaks this morning, so a 30 minute test drive was undertaken. Car ran great and I'll head out for a proper drive tomorrow. Malc