Glad to report that Customer Services have been in contact again and this time the login problem was sorted with Samantha's help. Thank you.
My husband has just had a very frustrating experience trying to access his pension details on Mercer. He has all the correct log-in details, they've confirmed that they have his correct email address on file, but when he tries to reset his password (as he hasn't accessed his account in the last 90 days) no email is sent so he gets no link, which means he's locked out of his own pension account. Disgraceful. Trying another message to customer services... we'll see...