OP. from the symptoms you describe the mech hanger was bent. Breakdown the cost into time and tools needed to do the job.
Remove the mech, 9/10 times you need to remove the chain, alignment tool on to the mech hanger, align it, I’ve had to remove the wheel before now because the hanger was so out out true I needed to put a G clamp over the hanger and drop out to add some stability before getting the hanger somewhere close before fine tuning it with the alignment tool. Mech back on, set the B tension screw, set the low and high limit screws, chain back on, check the cable is clamped correctly, set cable tension.
Tools: Mech hanger alignment tool £50-60, Allen keys £15, cross head screwdriver £7.50, possible chain tool and quick link pliers, possible quick link to rejoin the chain.
£25 sounds perfectly reasonable.
* Every mech hanger I’ve ever fitted needed to be aligned with the hanger tool. Doesn’t matter how much it costs, it’s how it sits in your frame.