I too replaced the impeller pump in the header tank myself with the inline pump attached to the boiler. Definitely worth doing but, may I suggest you do it when you have the Alde fluid changed as trying to do it on a full system while trying to avoid spillage added two hours to the job.
If I remember rightly the height of the impeller pump can be adjusted slightly. Try lifting it a bit as the impeller may just be fouling the bottom of the header tank stopping it from spinning. I’d also shop around as I thought they were about £80 new.
If you have to buy one and John can’t put his hands on his let me know as mine is in the garage at home.