I’m having to decorate the master bedroom, hopefully before Christmas. The window is a bay and the current curtain set up is on a curtain rail which has partially fallen off the wall. I can’t really fix to the ceiling as it’s lath and plaster and I don’t want to get into a mess, so what’s the options?
Rails or runners? Any specific recommendations for ones that are not too expensive or tricky to fit. I’ll be getting new curtains too, nothing expensive.

Depending on the angles of the bay, you could just get some normal plastic curtain rail (Swish stuff) as it has enough flex in it to go around the corners in most bay windows.

You just fasten a batten to the walls (assuming you can do this – you mention not being able to fit to the ceiling) and then screw the rail brackets into the battens.

I have just put some of this stuff up in a bedroom and it was really easy:

http://www.dunelm.com/product/swish-sologlyde-track-1000011673?cmCategoryId=34168

you can get rails that are designed to bend at certain points so they fit in bay windows. we had one in our last house…but the overall look was a bit shit. have you considered venetian blinds instead?

I’ll take a look at the batoning idea.
I hadn’t considered blinds, no, but it’s quite tempting. The window faces the sun rise in the morning. Plus there is a busy road at the front so curtains would probably help reduce noise, but blinds might cut the sun out.

Got to say the Swish rails are the easiest I have ever put up and once up work lovely. Another thing you might want to take into account is the weight of the curtains, they do different rails depending how heavy a curtain you are using, winter weight and lined are surprisingly heavy, apparently it can cause a rail to come out of the wall at night 😳

John Lewis metal ones bend into nice curves, good quality and cheapish. You’d mist likely have to put battens up though.

For cheapness – a swish sologlyde is good. Fits easily into bays and will hold up most medium weight curtains.
Metal rails can hold up a heavier weight.

Poles are trickier as you need to measure the bay perfectly to get the bend where you need it. The use of ‘D’ rings allow the curtains to pass over the special brackets on the bends.

You can fit some cheap battening up, paint it to be almost invisible and fit the rail onto this. The battening doesn’t have to protrude too much out from the wall,1 cm would suffice.

The poles can be expensive though. Where as a swish rail will almost disappear when fitted.

Roman blinds are good. However they will let some light in and ideally need to be fitted slightly proud of your window, again on battening. This will need to be 2cms away from window to allow for the mechanics. Blinds can also come with a rail style mechanism.