…are they any good?

This sort of thing…
http://www.passionauto.co.uk/products/autoleads-rma819-glass-mount-interior-car-aerial.html

I know I should replace the rear roof mounted one but I can’t be bothered to strip out loads of internal panels!

Depends on your windscreen. Some vehicles have reflective glass which plays silly buggers with internal aerials.

Don’t know about those, but I recently had a DAB radio fitted, using the windscreen film antenna supplied, which is practically invisible in the top corner of the screen, but still gets amazing reception, better than the FM reception I used to get with R1/2. I’ve driven from Chippenham to Weston-Super-Mare with clean reception except for two small areas near Bristol. Based on that, I’ve no reason to believe the one you’re looking at wont work, except if you have an anti-IR coated screen.