As far as I can see, there are no bolts on this unit that I can undo. I sprayed PB blaster on the outer retaining ring and then tried to pry it loose with a screwdriver, but to no avail.

Photos

I would rather not spend $200+ on a new pump if I don't have to. Thanks in advance!

Where have you been trying to pry the retaining ring from?  There should be a small hole in the casing somewhere.  Ideally you need to knock the ring around so the end is near that hole, then poke a small screwdriver/pick through that hole to flick it outwards.  I think I can just about see the hole on your third picture.

(PS - Can pick up a good used pump for £25 in the UK, are they particularly uncommon in the US?)

I bought a used pump and it wasn't good. If you have the time and inclination I'd get a overhaul kit, especially if you have a Ghia Focus/Fiesta with aircon (that is a PITA job).

Use a Torx 10 with a handle and use that to grind out any gunk from the hole. Then use a needle file or engineering scribe and run it round the retaining ring upper a few times. It looks like part of the main casting but if you look at the ends head on, you can see it is round.

Then do the bottom, and rag clean it. Then squirt penetrating into the ring top and bottom right round. Wait 5 mins, clean off, then butane torch the main casting just where it meets the ring. The idea is to heat the body so it expands away from the ring. Do not heat the ring nor the D plug itself. Heat it for about 2 or 3mins and keep the flame moving. It should not change colour but will small and burn off gunk so you'll get fumes.

Put an ice cube in the middle of the D plug and let it melt away. Use oven gloves, and shake the water off then use a hammer with a flat face centre punch or the T10 with the handle and drift the ring either way a little. Once you get it moving you can drift the gap (it's a good 15mm+) around to near the hole in the casting. Leave one end of the ring so it covers the hole and ends a few mm from the hole.

Then use the T10 and raise the handle up to point the Torx bit face down onto the ring through the hole in the casting. Lots of little taps then whack it. It should pop at some point and you can lever it out with a small flat face screwdriver.

The rest of the plugs can be scraped clean, pen' fluid and heat/cooled too and then they should be easy to unscrew. They are 8 and 10mm hex head (Allen key). They have a nylon seal washer that makes it harder to unscrew them.

Don't try to take the pulley off unless you know you need to (PITA).