I like Tesco Healthy Living Cumberland sausages. They're 1 syn each and come in a pack of 8 for about £2.50-£3 (I can't remember exactly how much they are)
I've tried sainsburys low fat sausages but I wasn't keen on them, I found them a bit dry.
I haven't tried hen, but the Heck sausages are supposed to be low syn too.
As for burgers, I make my own. I use 5% mince, make a few burgers and then freeze them. That way I know they're syn free
and I can add whatever herbs/spices I want to my preference