Update. First of all many thanks to all of you that replied.....much appreciated.
In desperation I decided to remove the sump again and go through everything just in case. Again it all looked fine, however having removed the sump I thought I may as well remove the roper plate as that was all that had changed. Before that though,I removed the filter to check the seal which looked fine, however it was almost flush with the top of the filter. I remembered that the old one protruded further, and luckily I still had it, So......refitted everything with the OLD filter seal and turned the motor over with no plugs and hurray after a few seconds the light went out. I checked that the pressure dropped before repeating and again got pressure. I've now ran the bike and it's back to normal. i.e. The light goes out instantly when it starts.
The new filter is a UFI as the old one was so the seal fitted fine. Unfortunately I didn't measure the old one before refitting, but the new one measures 5.5mm. The old one was at least 1mm thicker, possibly slightly more.
Yam350's post in Roper plate was marked as the answer