As the nursery and HV have already written letters requesting speech therapy, they presumably think there is some reason for concern which they can't deal with themselves.
Cherrytree, I speak from experience here. My son was a bright, lively child, who didn't talk until much later than other children. I was told not to worry and part of me wanted not to worry, but I just knew that there was something wrong. I regret waiting so long before I took action.
My GP arranged a hearing test at the local hospital. They do specialist tests on children, which confirmed that he had almost complete hearing loss in certain frequency ranges in one ear as a result of ear infections when he was younger. Armed with this information, I pushed for him to have an appointment with a speech therapist, but (as stated above) I ended up paying for private therapy, which helped a lot.
As he began to speak, he was almost unintelligible and spoke too quickly, so he needed to be slowed down. It was explained to me that he had lost time with thoughts going round his head, but not being able to express them. When he discovered that he could actually speak, he just blurted everything out. His hearing was still poor, so he couldn't give himself feedback as to whether what he was saying made sense. He had also lost the time toddlers usually have 'experimenting' with making sounds which sound like everybody else. His speech sounded like that of many deaf people and his behaviour was beginning to be affected, because he was frustrated that people couldn't understand him. It also affected his reading and writing initially, because he just wasn't hearing the same sounds as most other people.
The speech therapist helped him with sound formation and slowing down. Nursery staff and teachers (when he started school) were able to make sure that he always sat with his 'good' ear facing the speaker and they double checked that he had understood what was being said. It wasn't rocket science, but it made a big difference. He had hearing checks until he was about nine and now has no problems.
Cherrytree, it hasn't been my intention to worry you and I sincerely hope that your grandson's lack of speech is a temporary developmental delay. However, it could be something more serious and, if I were your daughter, I would want it checked out by professionals. I agree with you that group therapy sounds like a cop out.