OK, so a quick update. The batteries worked perfectly well in parallel above 85%. It was awesome being able to run sport mode 3 for so long without worrying about a fall in voltage robbing the bike of power. The additional weight also helped with the grip of the bike, making it much more reliable when leaning over into corners.

Then we run into problems!

The regen issue that @flyingelectric highlighted struck. Initially, it wasn't too bad, just very brief and momentary. However, it got so bad that no matter how many times I switched the bike on and off, I couldn't get the regen symbol to stop flashing. Eventually, I just unplugged the spare and completed my shift on the original battery alone.

When I got home, I tested the two batteries with the multimeter. The new battery is supplying power, the voltage had dropped about 3V from fully charged. But obviously the original battery is supplying more power and the two batteries are becoming unbalanced.

I have decided that the solution is to use the two batteries separately. I won't get a power reading on the new battery, but that's OK. I can use it until the power noticeably drops, then switch manually to the original battery and carry on from there.

If I can maybe find a bluetooth battery meter to get the voltage displayed on my phone, that might be an idea. And I still plan on hooking up a third battery in parallel with my second one. Hopefully that won't create any imbalance because I'll be using the same cells in the same configuration.

Disappointed, but not too disheartened.

@socobelfast

A solution can be to 1) disconnect the 4-pin connector on the original battery. That'll removes the battery status to a blinking 0%. That may solve the regen issue . If that don't work there is also option to 2) remove the bms for your new battery for discharge so that the bms is only used for charging your battery(because there may be issues between the two bmses while discharging).

Some info and thoughts that I have:

Why does your bike regen at 85%. At this level the newly build battery transfers energy to the original Super Soco battery. As this happens the original bms activates "regen" and as such terminates throttle input as a software programmed phenomenon.

https://secondlifestorage.com/showthread.php?tid=641

What it explains on the website above is that at certain voltage levels there is more energy and it may not be the same levels at the same voltage for these two paralleled batteries. Some cells have more energy stored in 3,3-3,5 voltage level spectrum and others between voltage levels 3,65-3,85 etcetera (calculating the square area for specific energy at each voltage level).

The second attribut is the power of a cell or a battery. If you would have had a less powerful battery that you had build you mightn't have any regen issues is one of my theories.

These graphs below shows what happens with the voltage level if you drain more power from a cell or battery:
https://www.richtek.com/Design%20Support/Technical%20Document/AN024

@socobelfast

Did you buy the original BMS? Or just a generic 17s one?

What is the size compared with original? I think it's not really half, but more like 2/3 from original. Right?

What is the final price? 

have you tested the bike with just the DIY battery?

i am assuming that the cells in the original soco battery are also 10A cells, not higher? baring in mind the original battery is supposed to be able to provide 100A, but the controller limits it to 40/50/60A (depending on model and drive mode) (those figures are only what i have gleaned from here and some basic power calcs)