I've noticed that some people have started covering their vegetable patches with black bin liners held down with a few bricks. Whats the advantages/disadvantages of doing this please.
I presume you mean now they have collected all the veg? Two reasons I can think of: firstly, it keeps the weeds down so less work digging in the Spring, secondly it stops frost getting into the ground and keeps the soil a bit warmer. I always feel a good dig over and letting the frost get into the soil is a much better idea and would never cover the veg patch in such a manner.
Yes that's what we do. A good dig over just before frost is due - the frost will kill a lot of weed seeds, then topped with horse manure and left until spring. Not everyone has access to manure though, especially on city allotments.
I used the black plastic method to clear beds on an allotment, bin bags don't really work you need heavy duty plastic sheeting. It worked really well, I also grew potatoes through black plastic, no earthing up and a really good crop. If it had been a heavier soil I would have just dug it and let the frost break it down. Once the spuds had done their work the soil was lovely