Thanks JP, I like cycling as common sense seems to apply where experience is lacking. I don't have a rack and don't want to buy one plus a bag only to find it doesn't work out though I'm not talking lots of weight - usually I'm ony carrying my lunch and a litre of drink, dry socks and a coat (leave home lunchtime, head home 10pm)
I've also been looking at jumbo saddle bags - better for balance and easy to remove but not so good off the bike, plus they block the seatpost and rack area so a rear light become an issue.
Or rack top bags which have shoulder straps and need a rack which lights could mount on, but they are a bit of a faff to take on and off...
(or inside the front triangle)
I think trying to convince a coat to stay at the top/front of one of them might be a bit of a challenge
) There are small front ones but they look narrow and generally tiny, though can't say I've looked into these too hard.