hey tom, Yes you need to upload arduinoisp code then attempt to burn the bootloader, I have done this many times aswell but with this chip it just doesnt want to work, Starting to doubt my schematic now. I even tried minicore on the arduino ide with no joy! Driving me insane lol
Will the Uno work to burn a bootloader to anything else, such as another Arduino? If so, then it would seem your circuit might be the problem. All of my bootloader burning has been to other Arduinos, not to just a chip, but my understanding is that all the chip’s ground pins need to be connected to ground, and all the Vcc pins, and AVcc, to the positive rail, and there have to be capacitors on Vcc - the Mini uses a 10uF and a .1uF, and there’s a .1uF from Aref to ground. The pullup resistor on Reset on the Mini is 10K.
But it seems to me that the first step, if not already done, is to make sure the Uno works to burn a bootloader to something else.
Edit: This is probably a silly question, but on your schematic what should be a pullup resistor on Reset is shown as a coil - L1. It is actually a resistor, right?