Survey came back with structural issues — walk away?

Had a full building survey done on a 1930s semi in Leeds (£265k). Surveyor flagged possible subsidence on the rear extension — cracking patterns consistent with foundation movement.

Seller says it's just cosmetic cracking from the 2022 heatwave but won't provide any monitoring data or structural engineer report.

We really love the house but this feels like a massive red flag. The surveyor said it could cost £15-30k to underpin if it is subsidence. Our solicitor is advising caution.

Has anyone dealt with something similar? Did you walk away or negotiate?