In our block of flats the corridor doors which are fd30 have had 20mm fire seals put on them recently to upgrade. I read that the seals should be 15mm. Does this mean that the door is no good?

The product data sheet for the particular model of door leaf will provide information about the exact fire seal requirements.  Generally speaking a firmly closed fire door is unlikely to be adversely affected by having seals that are 5mm wider that those specified.