I keep asking if she has contacted her friend and booked in yet. She keeps saying "no I need to do it, I haven't got around to it yet".
Why on earth would she be taking another friend and not you?! I would buy a present like that for something for us to do together, not for them and another friend- I find that really odd, might she feel the same?
I wondered if she's already used it with another friend or relative and, every time you ask her if she's contacted her friend yet, she hears this as you asking her when she's going to invite YOU to come along - which she may interpret as you being very cheeky, giving her a kind, significant gift - but effectively wanting to take half of it straight back for yourself.
If she has a DH or DP (and it's not a particularly 'girly' thing), then he would be her natural choice of accompanying person and nobody would think anything of it; but if she's single and planned on taking her DSis or a different friend (or has already done so), then she might be embarrassed to tell you, seeing as you insist on constantly mentioning it, that you weren't her first choice and therefore hoping to wait until it would have expired before lying and expressing faux disappointment that she never had chance to use her lovely gift from you.
Either way, I really think you need to let it lie now. If she genuinely does let it expire without using it (and it's still only about halfway through its very restrictive validity period), she's the one who will have chosen to forfeit her own present. You won't have lost £50 as, presumably, you would have otherwise spent that money on a tangible present for her anyway. Even if it is something that she genuinely wants, I fear that your endless going on about it will now have irreparably soured the whole present in her eyes.