YOUR BEE IS FAT ROUND AND FURRY
Your bee is probably the Tree Bumble bee, a fairly recent arrival in this country, hence you may not have met them before.
Tree Bumble Bees love loft insulation and they build a loose chaotic nest on it. They do not harm your building and when they have bred the next generation of Queens in June the will go and leave nothing. They will rarely use the same nest twice, though may return to the same loft next year.
BEST ADVICE: DO NOTHING!
picture © P.R. Harvey