Hi Tiges!
Mate, I'll give you my opinion here on this situation.
1) I really feel like this is a much better approach by your neighbours than many others I've seen. Drum mutes aren't cheap. They aren't telling you to stop altogether and they seem to recognise that you enjoy your drumming.
2) There was mention of finding it difficult to sleep by your neighbour. Well, maybe this person is a shift worker like me. I think it's a very fair, rational letter in this regard. Again, they are not telling you to stop altogether. They aren't being abusive.
3) Drums ARE actually loud. Sound properties are interesting and sound can travel some distance. Your local council do have services which can measure your decibel levels.
4) Unfortunately we also have a responsibility to behave within the bounds of what would be deemed appropriate. If you are smashing away at the drums for absolutely hours and hours on end on a Saturday.......well they kinda may have a point here.
5) It's a tricky wicket to navigate through in these circumstances. I totally understand this. My opinion on what you could do?....... perhaps use these pads. They are a terrific free gift for you. Perhaps play the kit with the mutes on for a few hours to warm up etc then take them off and play unmuted for a few hours.
6) I feel absolutely no spite from your neighbours' actions here. To me it's a genuine attempt to keep all parties happy. It's not all about what you want to do and it's not about what they want to do. I believe that they recognise this.
And as
@Yamaha Rider has mentioned, ultimately they haven't caused the initial problem.
I guess it's easy to predict that if you choose to ignore their suggestions and just think of yourself, you are going to get into some trouble I'm afraid.