Thanks Tanya,I did some obsessive googling since I wrote the post and found this:
"A positive result will generally stay positive for 48 hours. Some very faint lines may fade and become difficult to see, however. Note: sometimes a line will appear briefly as the dye in the test passes over the area where a positive result would show. This is not a positive result."
POAS ~ HPT FAQ
Hopefully just too early - I've stopped taking my temp because I was getting a bit too wrapped up in all I'll try again in a few days.