TrigpointingUK

Please leave a supplementary question on the forum. Your main problem is likely to be post-processing software, most of it is commercial stuff and very expensive. although, there are a couple of free/open source projects out there. whatever method of processing you use, you'd need to be able to ...

You're right in assuming that the OS coordinate converter is the only accurate one. All th consumer grade receivers and most of the online converters use a simplified algorithm, that typically has an accuracy of a few metres at best. The OS produce a (free) piece of stand-alone software called ...