I have been on another forum's entomology page and its suggested its a "Large Dark Olive female dun (Beatis rhodani)" mainly due to the time of the year as no other true olives about this early season.
Yes, Large Dark Olive is likeliest at this time of year. However, there are lots of very similar flies; for example, it could be a (rather early) Pond Olive (Cloeon Dipterum). Do you have a pond in your garden? I used to get Pond Olives in mine. The Large Dark Olive, and all the other Baetis Olives, have very small hindwings, but these are hard to see without a close inspection - I couldn't be sure from your (excellent) pics. It's not a March Brown, however, which has mottled wings.
Yes, definitely Large Dark Olive, I'd say.
The Baetis Olives: Large Dark Olive, Small Dark Olive, Medium Olive, etc, are very difficult to tell apart; they all look pretty much like the fly in your photographs. "Dark", Medium", etc doesn't mean much: even the Small Dark Olive is sometimes described as a "Pale Watery", and the size differences are not great. However, the Large Dark is much the most likely at this time of year, and is very widespread.
BTW, I don't want to pose as an expert; I like trying to work out what fly is hatching, but I'm really not much good at it!