In an attempt to gain a direct answer and to stop wasting others wasting their time I'll describe the whole episode.
I used to be a corgi registered plumber and heating engineer in a city. I never did oil work, always gas. Recently a friend bought a rural property and asked if I would help him run the oil lines to his Aga and boiler. I didn't really want to but owed him multiple favours so agreed. I genned up before I went, learning about regs, fire valves etc. When I came to buy the stuff the local merchants had all but a flaring tool and inserts, and told me that no-one used inserts for compression joints on oil anyway so there was no call for them and they did not stock them. I was dubious but only had this one chance to buy materials. I bought a tub of Rocol Oilseal hard setting sealant. I laid the line in plastic coated copper, both 15mm and 10mm. As Oftec had led me to believe that laying 15mm plastic coated copper underground was fine (see link above) I did this, but did not need to joint it. However, both 15mm & 10mm plastic coated pipe is jointed with compression joints (with Rocol Oilseal) in multiple places above ground.
I've been worrying about it ever since. I will not able to return to my friends house for a very long time.
I have inspected other lines since to see if historically inserts were used for jointing W table copper, and can't see any evidence that they were
Any pointers from the experienced to the likelihood of the line leaking would be very much appreciated.