Ask HN: What is the average salary for an iOS Developer at a UK startup?

I'm posted this using a throwaway account but I've been an active HN member for several years. I've been an iOS developer for several years and although I wouldn't describe myself as a 'rockstar programmer' I'm competent and a quick learner.

I'm considering job opportunities (I've worked freelance until now) and I am trying to work out the average salary an iOS developer should expect working at a small but fast growing startup in the UK.

Can anyone help? I know it's only one thing I should be considering but I've no idea what a 'fair' salary offer is.

tl;dr What is the average iOS developer salary at a startup in the UK?