My plan on this was to use Zedex CPT DPC in a single length as the tray and run it right along the party wall and across the external cavity. This would create a continuous tray that would ensure zero water ingress below the tray. I was going to get the brickies to rake out under the dpc so the lead flashing could be inserted.

The internet is awash with "bad idea, use pre formed trays ...) most of which comes from manufacturers or those selling said trays.

The exposed wall will be fully cladded and as such will be subjected to pretty much zero water.

What have others done here?

A continuous horizontal tray is fine for a flat roof abutment. That's the normal detail really; the preformed trays earn their money on stepped abutments under a pitched roof, where cutting and lapping individual DPC trays is fiddly and easy to get wrong.

Things I'd watch with a single length:

Get the fall right. It wants to rise across the cavity at least 150mm and be built into the inner leaf, falling down to the outer leaf. A flat tray just holds water and mortar snots.

Support it. Unsupported DPC across a 100mm cavity sags in the middle, so either use a tray support or have the brickies keep the cavity clean and the DPC taut while they build off it.

Stop ends where it finishes, and weeps at roughly 450mm centres, minimum two, even on the clad section. Cladding stops driven rain, but you still get condensation and the odd leak running down the inside of the outer leaf.

Laps 100mm minimum and sealed with the maker's tape or mastic if you can't do it in one length.

On the flashing, rather than raking out under the DPC afterwards, I'd get them to leave the bed joint below the tray open (or rake it 25mm while the mortar is green) so the lead tucks in, gets wedged and pointed, and the tray then laps over the top of the lead upstand.