Thanks for your reply, Al. I spoke with a couple of Lexus dealers (and Lexus Parts Direct) and surprisingly (?) none of them were really very sure what the specification required is. The comment from one was that the OEM one isn't an AGM battery - jury's still out on that... So, I settled for a Bosch S5 005 which is a 63 Ah battery and maintenance free, has a 5 year guarantee , and it's now fitted and so far so good all working. The Bosch isn't AGM either but by all accounts has technology to prevent it leaking.
Thanks for taking the time to reply. 😀