OP, you will need to go onto the Pony Clubs website, think its
www.pcuk.org, or just google it. I tried to find a branch in Rochdale, but you need a full postcode to do it. I tried with an OL post code, which I believe is appropriate to your area. Once found, speak to the branch secretary who is usually in the know and volunteer your services. They will love you.
Shame you are so far away. I have a lovely 20 year cob who adores his hacking, but mummy spends all her time working, so can only take him out on sightseeing tours at the weekend currently. Wouldnt want to be paid, just ride him a couple of times a week and do him at the same time - winners all round them.