Darren,

Are you sure that you put in contact with the resist the right side of
the transparency mask? That is, with the ink-side down?

It can sound like a silly question, but i've seen a lot of
experimented people make this kind of mistake. And your photography
looks very compatible with this kind of error.

Another possible cause of separation between the mask and the
substrate are the elevation on the edges of the resist generated
during the spinning, specially when you are working with thick
resists. Did you remove the edges?

Cheers,

Karim