I have also had a response - slightly more satisfying than Button's but that is probably because I asked for a refund rather than for them to honour the perks:
Dear Ms ******, thanks for your email.
I am sorry that the Passholder perks have been amended to remove the one free entry to other Legoland Parks. Thank you for pointing out the error on our website, we will pass this to our web team straight away.
As a result we are happy to refund the purchase of your annual passes. We will refund £87.00 to the account used to purchase the passes.
Please allow up to 7 days for this to appear in your account.
Yours sincerely,
**** ******
Customer Service Executive
Telephone: +44 (0) 1753 626182
Not necessarily the outcome I was hoping for (would rather have had the free entry but at least I got my money back. Just need to convince Tesco to give me my Clubcard vouchers back now as I also exchanged some of them to buy an AP.