I'm having trouble with mine when i go to pull release lever sometimes it works and sometimes not, don't think the catch is releasing properly.

I take it it's just a cable pull mech ? If so can it be adjusted to take up any slack due to wear ? Failing that i'll re grease the catch

Yeahs it's just a cable mate I'd check thats it's not being pulled or trapped on a bolt behind the grill on the cars chassis running across the front with the release catch just check the cables not being snagged on anything or how slack it is I had a similar problem :(   

winter -> salt from the gritters -> heavy corrosion. so probably both cable & release catch in need of grease

grease it up, try it a few times if it still sticks i'd get a new cable as it could be stretched, i don't think there is the option to tighten/adjust it i'm afraid, a quick look, it's about £20 for a new cable