It basically seems to be ‘spend £40 and get a voucher code for £10 off/spend £60 and get a voucher code for £20 (there may be other amounts)’ to be used within 3 months and as it’s a discount code rather than a gift voucher, it can’t be used with other offers or discounts.
The amount spent is cumulative so you don’t have to spend £40 in one go.
No free shipping unless it meets the threshold either.
There’s a thread about it here 