Thank you for the advice, I had already done as you have suggested in your last post...
I did go into the sub form, highlight all the textboxes, checkboxes ... etc (i.e. excluding Labels) and change the Locked property to Yes and the Enabled property to No. Then I went back to the sub form control to re-enable it, i.e. change its Locked property to No and its Enabled property to Yes.
What I had failed to realise was that the tab control page itself that the sub form is on was set to enabled = no - I have changed this now and everything is working fine.