I've recently change the pump on our 98 2.5. It's messy but straightforward.
If you still have the secondary air system it's probably as well to remove the parts behind the radiator to improve access.
After that, slacken three screws holding pas pulley, remove belt and tie it well out of the way, there will be fluid leaking. Remove pulley and the bolt holding the air conditioning pipe bracket.
Remove as much fluid as possible from the reservoir, put deflectors/rags to protect the ABS pump and A/C compressor, there will be a lot of leakage.
The pump is held on with two screws directly into the the engine and one through the bracket for the A/C pipe. I think there one more that holds the bracket.
There are two pipes to the pump, one flexible that is clamped and very hard to get at. It might be easier to release it the reservoir end and pull through with the pump.
The other pipe is retained with nut at the top right of the pump, slacken before removing all the pump screws.
From memory that should allow removal of the pump.
I do not understand why Vauxhall and Haines think you have to remove the A/C compressor and main electrical ground near the pump. Unless my 2.5 is different for some reason.
Anyone else tried it?