@SRE
Have you been here:
Planning Portal | 1-App
That should help, when you get the 'blank fee' click on the light grey 'Calculate' button and then it starts to ask loads of questions.
When you get to "Erection, alteration or replacement of plant or machinery"
The most important item here for large ground arrays is the area covered, since the fee is based on that! if you had a tracking array, you would need to work out the area that it swept over, if it was rows of static, then ONLY the actual area covered by the panels, the area in between if still left as grass would not be included, if you gravelled the whole area, even between the rows, that you have to include that.
If its going on a an existing flat roof with angle mounts, you'll generally need to include the whole roof area.
Hope that helps
I just did this for a farmers roof on his packing shed, (10m x 30m 28kWp) and the fee worked out at £335.
Though that isn't the real cost, that just the fees!
Now you need to take into account preparatiopn of drawings, correspondence, communication with stautory undertakers, discussion and liason with the officers, attending the planning meeting (that is if you really want it to succeed...)