Thanks for the advice guys. I've been playing around with my shot preparation this evening which has left me feeling a bit wired but has been fruitful.
I think I have become a bit complacent lately as my shots were starting to become consistent, but thanks to the advice I now think I've been flushing the machine too much and cooling down the brew temperature. I also tried giving a longer pre-infusion which seems to have done the trick.
I'm still not convinced with my roasting. I've managed to dial out the sourness as described above with the Costa Rican I roasted last night but despite it now having balance it seems a little nondescript and there is still an acidic aroma which thankfully no longer transmits to the tastebuds. Maybe I'm now brewing too hot, masking the acidity but losing the more delicate flavours. No more messing with it tonight though, I won't sleep until tomorrow lunchtime!
I'll try all your roasting tips over the next few days and let you know.
Cheers