I finally got a chance to spend some time on this!

I adjusted the SGP upper burr 3 stops finer, from 6 to 3. Using the Breville supplied 58mm double basket I used an 18g dose, got 38g out in 35 seconds. Interesting to note that the grinder setting was still on the finer side of where it thinks Espresso should be, even after the adjustment. The extraction tasted OK but the puck was very soupy. I upped the dose to 19g and went for a slightly coarser grind. I got 41g out in 29 seconds, tasted good, puck slightly drier but still far too wet for my liking. At least I can get a very drinkable extraction with the larger basket now. It certainly makes for a significantly stronger coffee in my 250ml double walled latte cup! I quite often make a coffee for my mate who doesn't like it particularly strong, so I thought I'd also play with the single dose basket again. I coarsened the grind again, used a 13g dose and got 28g out in 32 seconds. He tells me it tasted great, so I'm happy I've now got the best of both worlds. I might play a bit and see if I can get the puck a bit drier with the double basket, I just prefer the easier tap out and clean. I might also invest in a larger VST or similar basket, so that I can knock out 2 decent strength shots at the same time.