I have a smallish amp in the boot and the wire that runs directly from the battery to the boot which was already in the car does not appear to have been fused.

The amplifier is an Alpine mono amp which produces about 150w.

The Amp has two 20a fuses but to be extra safe I want to stick one close to the battery. What fuse rating should I use ? What have other people used ?

Use a 30 amp fuse mate.

I always usually match the power wire fuse (close to battery) to the sum total of the max possibly current draw of the amp(s) I'm using. So in your case, i'd go with a 40amp fuse.

If you use a 30amp fuse, it may be fine, if you're not running your amps over say 80% of what they can kick out. If you run them hard, you may have the fuse near the battery blow, which is no biggie, other than you'll keep having to replace it.

If you're not running it too hard, it may never draw more than 30 amps, hence you may never have a problem, up to you mate - either way it will be safe.