So we finally saw the physiotherapist today, who was a very nice woman. She was pleased to tell us that Grace's case is not very severe. The muscles are the same size and she felt them and said that both sides are still soft and therefore flexible.
Her advice was that since the case is not that bad, we should work on her neck at home rather than start regular physiotherapy sessions. She said the sessions would be very stressful for a young baby and with only around 20 minutes of exercise at home per day, we should be able to correct the problem ourselves. She did say it might take a little longer though. She told us that we merely have to encourage her to look and lean the other way (right) by getting at eye level with her and making eye contact, getting her to follow us by turning her head. We should also gently turn her head whenever possible.
What I'm going to do is work on this daily with Grace, and if I feel that it's not going well, or if I feel uncomfortable or confused about something, I'll take her to another hospital to get another opinion. Fingers crossed.