I have had a surprise invitation to the evening wedding reception of one of DS's friend's parents.
It is to be a fairly small, low budget affair at the local community centre. Very nice people, but I've never seen her wear anything other than baggy tracksuit and t-shirt. (not sure any of that's relevant, but trying to paint a picture)
I've never been to a DIY wedding reception before, but my usual strategy when unsure what to wear is a black, jersey dress from Coast, which I got in the sale last year and has been a real bargain, as I've worn it loads. I think they described it as a cocktail dress, but I'd say it's a bit more casual than that. Lovely soft material, handkerchief sleeves to elbow, lowish scoop neck and full skirt. Does the fact that it's black rule it out for a wedding, even an evening only do?
I wore black to a wedding just last week - I love black, but it was mostly because I can't get into very much atm. I put a gold(not blingy gold), light, shrug style cardi over, just to lift it a bit. It was a low key late morning registry office affair followed by a low key meal.
Would only possibly remotely be a problem if you were going to a church wedding with very very traditional people hosting it/being the bride and groom. Though all black plus black hat with huge veil is maybe a bit too funereal unless brid and groom are goths.