[helen842000]

If it was me, I would call and get straight on to their finance / billing team. Just chase it until they realise that paying is the easier option. Not in an angry way, more just stay on the phone until it's sorted, 'let's wrap this up today' kind of attitude.

It's too easy to ignore your e-mails and then there is such friction with replying after they have ignored you several times and think they are getting away with it.

Being based in the UK, I will follow up on your behalf if you want. I like getting this kind of thing sorted.