Hello there,
I believe you are making a bigger deal of it than it is. But don't worry, it's normal.
For example, there is only one 300 NM VFR XC required - is this actually the norm or would most hour builders be expected to have logged say, 3 or 4 of these?
No, that's not expected. You simply need ONE such flight. If you do more than one, that's great, more experience and fun to you but don't feel like anybody expects that.
How do you log your hours? Should I keep some sort of flying diary where I can record details of flights, rather than just the bare minimum log book entries? What should be included in this? Is there an official document for this purpose?
If you are doing your PPL I assume you have a logbook already, don't you? Just keep logging as usual.
You can keep a diary of sorts if you want, but that's really just up to you. It's not a bad idea to comment on your flights, make notes of events or learning points that may help you in your training, but those are for yourself and nobody needs to read those! The logbook is what really matters.
Any other unwritten rules about what should be part of the hour building process, or suggestions that would be useful?
My suggestion is to try and fly to different airfields and as far away as possible. I've seen many time builders just drilling holes in the sky in the immediate vicinity of their base airport, this is boring and pointless in my way (but still legally acceptable to meet the requirements!). Of course, there are other constraints that may keep you from flying long trips away from base: weather is a big one and if you intend to time build in Scotland I assume you won't be able to get very far. There's also the ATO who may not want you to take an aircraft away for several days, or to fly too far away. Within the limitations you are given just try and make each flight different, that's my best advice.
Also keep your general handling skills current. You may want to fly one hour with an instructor every 10 hours solo, for instance, and practice general handling, steep turns, stalls, PFLs, circuits, crosswind landings, short field performance etc.
Finally, don't get too complacent using moving maps (SkyDemon) for planning and navigation because examiners really don't like them. It's archaic but it's the way it is. Use a moving map as a backup and for safety but try and keep your dead reckoning skills sharp.
Last advice: Scotland is a really crappy place for VFR flying due to WX, as I'm sure you'll know. There will be many weeks where you'll hardly log any hours. Explore the possibility of going to the South of Europe (or maybe the USA?) for hours building.