Updating selected groups from a recordset

I have a table with three text fields: Photo (unique), Sequence, Sort

Photo always has data, Sequence sometimes has data (and each
individual piece of data will appear at least twice), Sort has none at
the moment

I need to update the Sort field based on numbers extracted from Photo.
This is easy for the non-sequenced records

For the sequenced records, I need to sort by Sequence, then by Photo,
grab some data from the FIRST Photo in the sequence and put that in
the Sort field for all records that contain that particular sequence
only. Having done that I need to move on to the next group of
sequenced records and repeat the operation, again putting data
extracted from the FIRST photo in the new sequence into the Sort field

It's Sunday evening, I've been working all weekend, and I'm having
serious brain fade. I would really appreciate some pointers. I want
this to run in code from a button

Many thanks

Re: Updating selected groups from a recordset

A good night's sleep has made it all clear!