I've just got an A4 Cabriolet and want to change the number plates. Hope it's not a really daft question but the rear number plate was screw on - no problem. The front one looks like it's just stuck on(?) no obvious screws from the front. However when trying to pry it away, it simply isn't budging and I don't want to break anything... It isn't some kind of holder is it?

Hello Mark,

Thanks for being in touch. 
No such thing as a daft question. Sounds like it’s simply! stuck on, but the double sided tape used to stick it can secure it very (in caps) firmly, as you have found. 
To aid its removal, use a hair dryer to heat the outside face of the plate and to soften the adhesive. Try to start from one end and carefully but firmly apply heat and pulling power. Once it starts to come away, then you are usually winning. Removing the remaining double sided tape? Very difficult, so generally better to simply add new tape in the gaps, and leave the old remnants there. 
Kind regards,

Gareth. 

Once you have softened the adhesive with the hair dryer and it still refuses to budge, try cutting the adhesive with dental floss or fishing line to 'saw' through.

Best of luck, absolute worst scenario is the plate snaps.