Pretty sure the alternator has gone on my polo gti.

Is there a way of telling whether its the 70amp or 90amp unit?

It should be stamped into it i would think.

If they are both the same fitting then I would go for the 90 amp one.

HTH

I have one sat here if needed

Cheers. I had thought that but wanted to check.

I also wondered if the alternator would be higher on climate control versions, if it takes extra power.

I've got two from GTi's (one's not putting out power though), They're both 90A.

That doesn't answer the question i realise, but it could perhaps suggest the 90A was at least quite common.