Thanks all. I've decided to only purchase KeyNote for now and have done so via the App store.
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I just found this! What a saving. Do you think they can be trusted? I don't see why not, as the purchase is made via PayPal.
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I'd say if you want it cheaper keep looking at HotUKdeals website as occasionally iTunes* vouchers come up from different retailers with varying discounts.
(like this one http://www.hotukdeals.com/deals/25-itunes-card-for-20-superdrug-1331572)
* I assume you can use iTunes gift cards for app purchases.
I just found this! What a saving. Do you think they can be trusted? I don't see why not, as the purchase is made via PayPal.