NJ,
In most constituencies it is clear cut. Cadiff voted Remain, so Cardiff MPs should adhere to that.
My own area, represented by 3 Labour MPs voted to Leave, so they should support the invoking vote. But all 3 of those MPs are Remainers and may vote against their constituents wishes, which is wrong.
Leave won the Referendum, so if MPs in those areas vote the same as their constituents, then invoke will win. Two thirds of MPs were Remainers, but Leave won, so it is MPs not properly representing their electorate that may cause a bill a problem.