There are a lot of solitary bees, Retron, identification made that bit more difficult as males and females can be somewhat different [size/colour] in some species. My first thought is "solitary" and "mining bees". We've had quite a few here. e.g. Tawny Mining Bee and Hairy-footed Flower Bee.
The mining bees make those small earth "volcanoes" on bare patches of ground. You may well have noticed these at the Wolf Centre and assumed they were worm casts. Could yours be the Ashy mining bee (Andrena cineraria)?
Now that's interesting - yes, even though my phone's zoom lens mangled the detail, that mini bee matches the look and description of an ashy mining bee. I'm ashamed to admit I didn't realise you could even find such small bees in this country, my knowledge started at bumble bees and ended at honey bees!
(I have a nest of bumble bees in the garden again this year, albeit a few metres away from last year's effort).
I may well have to poke around my garden a bit more closely this summer... I've a feeling there's a heck of a lot of "minibeast" life that I simply haven't noticed, and the same will apply at the wolf centre too! (Albeit there, we're generally focused on much bigger wildlife, such as the roe deer which burst from a hedge yesterday... made a change from the odd-looking muntjac deer we usually see).