A bit of an update on this.
I wanted to use the car for a 350 mile round trip over the weekend so had a quick look last week. I came to the conclusion the front pads were very worn comparing them to the new ones I had bought.
I bought one of these to push the pistons in properly.
https://www.amazon.co.uk/gp/product/B08594G73B
I discovered that one of the slide pins was sticking on the opposite side to where I thought I had the problem.
Using the ratchet tool, and with the bleed valves on the calipers open, I was able to push the pistons both sides back easily.
I replaced the slide pins on the side that was sticking and fitted new pads both sides.
The brakes work fine and stop the car without pulling but both sides feel as though they are binding very slightly. This is enough to make the wheels warm but not boiling hot on a long drive.
I have compared to one of my other 900 series cars which had its brakes serviced professionally recently and that one is not perfect but the front wheels on it will spin a little bit longer before they come to a halt.
I’m wondering if this will sort itself out as the new pads wear and bed in or whether I need to do something else. I didn’t replace any other parts. The Delphi brake hoses I bought would be long enough (thanks Classic Swede for mentioning the need to check this) but I managed to convince myself they had the wrong end on them not realising the infamous 1329693 connection adapter is from M10 male to M10 male, as described here.
https://www.volvoforums.org.uk/showthread.php?t=316504