Have to hold lock button up to open doors from inside

When I open either of my Defenders doors using the interior door handle I have to hold the interior lock button up to open the door. Both doors are like this.

Any ideas how I can sort it so they open normally again?

Only from the inside? they work fine from outside (assume so otherwise you'd be locked out...)

I'd take the door card off and take a look, I assume the link that connects the inside handle may be getting stuck against the lock connector, by taking the card off and taking a look it will hopefully be obvious what to do

Yep all fine from outside. Just from the inside.

They keep doing it, I strip the door down, grease it all they are fine for a while then start doing it again but I can't work out why?

i belive its something to do with a little spring in the lock mechanism. you can buy them as a pack ill see if i can find the link on ebay.
http://r.ebay.com/wklzoX

Is that the same for the Defender? It's says Disco & RRC on the ad?