Tried a 3.579545Mhz XTAL as shown in the PIC32 family design reference with all the correct values for the baud rate generator and the wifi could not get a lock at 115200 baud, replaced the 8Mhz XTAL and upped the clock to 20mhz and all ok again. 

I have since done a bit of research (wikipedia was quite good here) and it looks like 7.3728MHz crystals are the way to go as these divide down to 115200 exactly, luckily they are also plentiful and as cheap as chips, I just ordered 10 for a dollar, delivered to my NZ rural address!  (How do they even do that, someone is really losing out somewhere in the chain and it ain't me!)

PS the saleae logic analyser is magic for finding things like this as you can set the channel as a 115200 baud serial and it will show the data stream gradually going out of frame, I have a feeling I'm going to get my moneys worth out of that analyser on this project!