@User: But yes, if you are looking to reduce noise, then put a deep nwell around the circuit and double-guard ring it with one guard ring on the nwell border of the deep nwell. For the deep nwell to be effective, you must completely isolate substrate ground (noisy) from your analog ground (quiet). The analog ground connects to the pwell inside the deep nwell area.

I've currently got an nwell below each pmos device. Should I add another pwell below and a deep nwell, leave the deep nwell out and just rely on the surface nwell, or remove the surface nwell and just use the deep nwell (don't think the last one is possible?)

@Trevor Clarke: You can't put a pwell under an nwell. The nwell itself is noise isolation. But nFET devices over the substrate are subject to noise. So the deep nwell is put down to be a barrier layer between the substrate and another P region above it, which is the pwell. In other words, you can only use the deep nwell layer to noise-isolate nFET devices. The pFET devices didn't need it in the first place.