An electrician friend also suggested just testing with different power outlets and keeping the power and av cables as far away from each other as possible, not coiling them, and running the laptop on battery power (which I'd tried, and it didn't solve the problem.) I'll try all at once and see how I go.

The cables seem to be pretty good quality, heavily shielded, chrome connectors etc. I suspect the easycap and/or its cabling, but moreso the teansformer might be the source. The issue didn't occur with my cheap, Australian vcr and composite cable