So I had a rethink about the Sideways RAM and I came to the conclusion that possibly there were some timing issues causing the instability. According to this post on stardot someone had managed to piggy back a SRAM on the internal ROM without the issues I was seeing.
With this information and due to the fact I was using a 62256 (32k x 8) I figured I could actually make the cartridge without the 74HCT139 as ROMQA (Edge-A16) could be connected directly to A14 of the ram chip. /OE (Edge-A2) could be connected to /OE and /CE of the ram chip. R/W (Edge A4) could be connected directly to /WE pin of the ram chip.
The easiest way to to this was to cut the 74HCT139 completely off the board so we can use its VIAS to make the connections. I had to restore the track I had cut on Edge-A4 (Edge-A11 is still not needed). I had to make a couple of new cuts to disconnect the A14 on the ram chip from be wired to Vcc.
Anyway this is the front of the PCB from the new version.
And this is the back of the PCB where the real work with the wire links and cuts occurs.
And just to show this is all working, *ROMS command issued just after switch on.
*ROMS command issued after loading 2 roms from disk.
New welcome screen after break is pressed to initialise the new roms.
And just to check roms are executing code the *HELP +1 command that Pres rom adds.
Im pretty happy with this project now. My only regret is that Edge-B10 is not populated on the cart (as no unused pads are). If so I would have wired it to the spare socket to provide ROM 13 and not wasted a socket.