I didn't realise I'd need to have qualifications to run a small sprayer on just a couple of fields of my own. We're not a commercial farm. When I asked about a suitable product at Mole Valley, they didn't mention needing any certificates to purchase the weedkiller.
Hmm, will have to investigate. Either that, or just do what I've done with the other field and wander round with a backpack sprayer and take a lot longer over it.