I wasn't brave enough to attempt cutting his hair myself, I think I would have made a real mess of it.
She is a local dog/cat groomer, but she has her own long haired guinea pigs (one of which was the biggest pig I have ever seen - he was a real beast of a pig!). I dropped him off and picked him up 30 minutes later, so I'm not sure what she used to do it - but I will take him again in a few months so will ask her then.
I thought she was very reasonable with her charges, it cost £10.