Morning Godisa - thank you so much for reply. You're right, things always do seem better in the morning; it's the nights when I really feel like giving up.
In answer to your questions:
Re qualified support worker - I've seen the breastfeeding support worker from our GP surgery. She was really helpful and he was latching well when she last came - think she's coming back out on Monday. It's since she last came that he's started fighting and refusing to latch.
Re positioning - I'm strill trying to find what works best. I have very large breasts which makes some positions more difficult than others. He seems happiest laid on his back on my lap with his head turned to the side to suckle - not exaclty traditional position, but hey, whatever works! Have also tried rugby ball and traditional cradle hold.
Re latch - he can latch well and I'm reasonably confident I now what a good latch feels like; it's getting him to stay on that seems to be the tricky bit.
Re tongue-tie - hasn't been checked for this to my knowledge and I was wondering whether I should ask, although since he can latch well and feed when he wants I don't think it's a problem. I think he's confused and maybe a bit lazy! I know I wanted a laid back baby, but this is ridiculous 
Re frequency of feeding - at night when he's on formula / EBM bottles it's around every three hours. Through the day when he's on the breast it can vary from 45 mins to a couple of hours.
Re pumping - I'm trying to pump after he feeds every time as I want to build up enough so that he can have EBM rather than formula; at least if I can do that I'll be down to only two types of feeding to contend with
I've had a read of some of the Kellymom articles and at the suggestion of the support worker have done a couple of duvet days with us lying around in bed having skin to skin etc. Have tried feeding lying down without success, but will try it again.
He did latch on for a bit of breakfast, for about 10 mins, after a fair while of fighting and resisting. Once he's on, he needs constant tickling or other 'motivation' to keep suckling otherwise he just nods off.
Anyway, 
for the response - I think you're right that he's confused and I need to pump more and persevere. I'll crack on through today and see if tonight we can at least work with only EBM rather than formula.