Hotpoint TC71 condenser blows cold - changed thermal cut-outs?

My drier is tumbling but is blowing cold air. I've changed the thermal cut out switches on the back, but it made no difference. I have used a multi-meter to check continuity across them and across the element which also flukes out fine.
I read on this site about another cut out switch just inside the front door which I've located but just can't work out how to get into it.
Just after I found it and pressed it, the drier started blowing warm again but only lasted a few minutes then started blowing cold.
Can anyone help with getting access to test this or advise if the problem might be something else?
Thanks, Lorraine.