Draught proofing strips are dirt cheap. Go down to B+Q and get some, and a cheap curtain rail. Then go to Oxfam, get an old curtain and hang it on the rail and that will stop anything that gets past the draught excluder.

Reminds me, I really need to replace the strip around our front door. It's about a decade old now and part of it's come away.

(How thick is the loft insulation? Most people over-estimate what they have. It's worth actually going up there and measuring it.)