I spotted this insect flying past me and alighting briefly a few yards away. I thought it must be a hornet as it was very large (about 2cm long) and had that classic hornet coluration of yellow and rusty red, with amber wings. However closer inspection suggested it was actually a hoverfly of some sort, and a lengthy internet search showed it to be Volucella zonaria, which does indeed mimic hornets and is apparently the largest hoverfly to be found in the UK.

It seems they're not found especially widely, being confined mostly to the South and South-East of the UK.