Alwyn
I agree with Ashwin. I'm in the UK too, as you probably know already.
And I know HSBC - used to have an account with them long time ago.
To add to, and also agree with what Ashwin said, these are my thoughts:
Make certain the HSBC app is compatible with the version of iOS on your iPhone. Some Banks are very fussy about iOS or OS versions.
Check again what the HSBC criteria are for a password - 6 to 9 letters and numbers, OK; but it may not allow duplicates - in other words J141K etc would not be allowed because there are two 1s; ditto letters. They may not allow consecutive numbers or letters. They may specify that one or more letters must be upper case, the others lower case.
I don't imply any insult or ignorance on your part - I just know that some Institutions are very fussy!
Then, as Ashwin said, typing on a SE keyboard is a challenge at the best of times. I know what I'm like on the "big" phones or even my iPad Pro 10.5 inch!
And as a BTW, I have to say that 6 to 9 characters is woefully inadequate these days, especially for something as crucial as Banking. Not your choice, of course; but some Banks just haven't got to grips with the latest security. Look at the TSB scandal.
For example, my Apple ID is 19 characters long. I'd want something similar for a Bank.
Ian