Due to changes in my working situation, my annual mileage is going to be substantially reduced.

I took out my current policy with an annual mileage of 23000. 

However, my commute reduction will knock off nearly 7000 of that from the end of July till policy renewal early October.

 

Is it likely to be worth notifying them of this, in terms of a reduction in premium, or would the "admin fees" make it a case of me handing them more money for nothing?

Obviously, going forward in October, I'd anticipate a smaller premium, but probably wiped out my price hikes I'm reading about in the industry.

Your current provider is the only one that can answer that for you accurately I'm afraid as each insurer's viewpoint is different.