Some HSBC branches with the same STD code seem to have similar fax numbers. I have found the following in Milton Keynes (ie with 01908 numbers) listed on HSBC's website (see here):

207 Queensway
Bletchley
Milton Keynes
MK2 2EE

Fax: 01908 455 099

406 Silbury Boulevard
Saxon Gate West
Milton Keynes
MK9 2ND

Fax: 01908 455 199

53 High Street
Newport Pagnell
MK16 8AU

Fax: 01908 455 299

19 Midsummer Place
Central Milton Keynes
MK9 3GB

Fax: 01908 455 325

Note the similarities. All the numbers start with 01908 455. I have noticed this pattern with HSBC in other STD codes, so I think that we can safely say that they have a whole block of numbers at each of these addresses starting with those digits.

The Midsummer Place address will therefore be 01908 4553xx. This page lists Robin Penfold as the business contact at that branch with the number 01908 455317.

If you are looking for someone in particular, you could try some of those numbers now (ie when they're closed!!) as they (especially personal extensions) may go through to voicemail which gives a name. If you do, make a note of what you find and come back and let us know. I also suggest that you withhold your number with 141.