I have just spoken to someone from T-Mobile about the promotion.
Apparently if you joined on the "free evening and weekend text" promotion, you get this for life and don't need to opt in to topup Friday because the offer isn't as good - you need to top up by £10 every Friday to get the texts.
I mistakenly opted in to topup Friday and didn't get my free weekend texts for some reason (T-mobile couldn't tell me why but they refunded me with the credit I spent texting at the weekend).
Anyway to cut a long story short, the lady I spoke to put me back onto the evening and weekend text promotion AND gave me free evening and weekend texts for the rest of this month, even though I should really have topped up by another £10.
I mentioned to her that IMO T-mobile should have made it clear that Topup Friday was only by default for new customers and those of us who are on the evening/weekend promotion shouldn't opt in because we'll lose our deal. I also mentioned that it isn't clear whether you have to top up EVERY Friday or just one Friday every six months. She promised me she would escalate the comments.
I came away from the call a happy bunny.