Neighbour is building a side extension to boundary, so should the foundation trench be shuttered on the boundary side before the concrete pour ?

This surely depends on the ground conditions? If it is really firm soil then the soil itself will 'shutter', no? Are you looking to avoid 'boundary creep' (or whatever one might call it)? If so, seems there are 2 options:

1. If it's really firm ground: Dig straight and true. Fill.

2. If it's not firm ground: Set up shuttering (N.B. This could well involved excavation on your side in order to ensure solidity - are you 'geared up' for that?): Fill.

Tricky. They are supposed to be building on the edge of the foundation. Have machine dug a deep trench certainly 1m deep and 600 wide along the line and also hand dug out clay etc. from above. Inevitably the side is not completely straight and true.

Excavation on my side is undesirable, there is only a 900mm gap and my own drainage runs down it.

They have been hacking at the sides a bit.

20% of their brick inspection chamber hanging above the trench and clay pipe collar chipped off (plan to repair with cement ?)

Gas have told them they can't come for 8 weeks to relocate the gas pipe & meter but apparently they can dig around it.

Have self certified for lateral drain which is just over 0.5m from front of extension (but they are building a platform and steps forward and over) - considered £300 for a build over agreement as unwanted cost.

Not entirely reassuring.