Lots of advice here and understand that you find it stressful, but a word of caution about rushing off to contact the council at this stage. Well done for speaking to them today and try and continue dialogue if you have queries.
I’d suggest that you have a look at the Planning Portal and click in the interactive house to check if what’s being built could be under permitted development. As already mentioned, there’s no 50 clearance requirement under planning regulations.
On using the summer house for their business, you’d need direct information rather than supposing/assuming. Here’s the thing (and I work in this area), if you contact the council and they send out an enforcement officer (some won’t due to staffing/resources challenges), they will need to state what they are investigating and the information in the (your) complaint. They shouldn’t disclose who has complained, but it’ll probably be obvious to your neighbours that it’s you. One of three outcomes:
1- the officer will identify no evidence of a breach of planning
2- the officer will identify a breach and probably require your neighbours to submit a planning application
3- the officer will identify a breach, but decide that it’s not worth taking action
On the use of the building for business, without evidence, or until the business use becomes established, or neighbours confirm, there’s not much the council can do.
But, your neighbours might think you’re a busybody, trying to cause problems, and acting behind their back. Whatever the outcome, remember that you’ll still need to live alongside your neighbours and don’t want to switch stress about what they’re building with stress about their reaction/behaviour towards you.
Most people don’t intentionally breach the planning regulations and I’ve witnessed many cases that have turned incredibly nasty because of perceived/actual accusations, and too many assumptions instead of conversations.
Noise, dust, inconvenience etc can bee a right ball ache, but in reality the best thing is often to look forward towards it coming to an end. So, do your research and have a conversation is my advice.