Both options sound a bit unusual, to me at least.
There are variations but a common US ESPP is to buy every 6 months, at a 15% discount and at the lower of either the start price or end price.
Here it's quarterly and that option element of buying at the lower price may not exist? If it's purely a 15% discount, that sounds more like a BIK to me. The option of buying at the lower price is why the usual ESPP falls under RTSO (Relevant Tax on Share Option).
The buy one get one free isn't one I've heard of, it should be more valuable but
- I'd make sure there's an option to sell the shares you bought before 12 months if you need the cash or you don't like the outlook for the company.
- also you'd need to be confident Revenue are happy it's a share option, i.e. Revenue don't expect 52% of the free shares when granted as opposed to when exercised.