Many thanks for the advice. I have taken the consensus view and notified my architect of the breach of copyright. He's grateful that I've pointed it out and is taking the matter up with the new applicant.

I successfully applied for planning permission last year to convert a townhouse garage into habitable space. Whilst often no planning permission is required for this, there are special parameters around parking on my street which meant I needed to apply. The street is a row of about 20 identical (not mirror-image) townhouses in a row. As part of my submission, I engaged an architect to draw up plans, costing around £1k. A neighbour on the street is in the process of selling their house and the prospective owner has just applied for planning permission to do the same work. The planning drawings are identical to mine, other than that the architect's details have been hidden with some different text - it looks like that was done with a white box in powerpoint or paint. This is essentially theft of intellectual property although I feel it may be hard to prove. Do I have any leg to stand on to seek compensation? Thanks for your help!