Done a bit of research

I've currently got a PHD Minimus. Four options (at the moment), two come in around 680g and two come in at 830g. The cost varies from £116 to £290

The weights include the Minimus except for the option of the Minim 400 which is a replacement bag. The cost does not include the Minimus, i.e. it's what I would be spending.
Option 1. Use a PHP Filler bag to go with the Minimus. Combined weight of 690g, cost: £219
Option 2. Get a PHP Minim 400. Weight: 670g, cost: £290
Option 3. Add a Cumulus Quilt 150 to the Minimus. Combined weight of 825g, cost: £116
Option 4. Add an Enlightened Revelation quilt to the Minimus. Combined weight of 835g, cost £158 (not including any potential duty)
Current thoughts are: option 1 feels neat but also expensive for what it is, it's actually more expensive than the main bag! Option 2 looks like the least flexible option, it would be only useful for spring and autumn but would mean that there's less wear and tear on the Minimus. Option 3 is flexible, more so than option 1, and could be used as a summer bag in it's own right. 160 grammes heavier though. Option 4 has the same advantages as option 3 but a bit more expensive.
One point I haven't considered is the extra bulk any of these options.