Marshall 10 watt combo clone

[Bushbaby]

All,
I hope you are all keeping well.
I am now determined to get on with this project having virtually all components, case & chassis, Transformers (thanks to Ed Dining) etc.
There is a modification that I want to incorporate to the circuit which was not included (due to original costs of the unit) which is to add a second ECC83 to the input so that the major issue described as follows (taken from Guitar.com):

The issue is that the volume and tone controls are situated between the screen grid resistors and the grid of valve. What’s more, the two channels are joined by a couple of 470k mixer resistors, which attenuate the guitar signal before it even reaches the valve. The volume and tone controls of valve amps – including most Marshalls – should come after the first valve stage.

I've added the relevant part from the modification that was undertaken (screenshot attached     ) and another copy of the original circuit diagram      . I've highlighted in yellow an area I'm having a problem with.

Can anyone please offer help/advice so that I can get started soon.

Thanks in advance

Rob