I think it's best to agree to disagree Malcolm, as it's clear that we have very different views on the EU and how it operates.

You take a very British view that if it's not written down it can't happen and that the EU will do things by the book. I think that the EU interprets treaties and the law to suit itself and it regularly turns a blind eye when it suits.

Budget deficit and debt rules spring to mind.

Take the Irish border for example. The EU say that a hard border or any sort of infrastructure at the border will contravene the Good Friday Agreement and basically blackmailed Theresa May into agreeing the 'backstop' where NI stays in the single market and customs union. Thankfully she later backtracked and said that she would not agree to a border down the Irish Sea effectively splitting the UK up.

So on one hand a hard border between ROI and NI would contravene the GFA, but they don't seem worried about the fact the splitting NI from GB would also contravene the GFA.

Personally I think putting a border down the Irish Sea would be far more damaging to NI than a hard border across Ireland, but I'll leave that to our Ireland expert for comment!

Because there is no legal basis to impose new rules on a member state without their agreement. If all EU28 and the commission have agreed Art50 is revocable, then how can anyone attach further conditions? The Uk just says no to the conditions, and revokes anyway.
If the UK gets to a point where it wants to revoke A50, we will be weak, humiliated and desperate. If the commission, the EU27 heads of state and the EU parliament present a united front and decide that the UK can revoke A50, but only if conditions are met such as adopting the Euro, we would have little option but to comply. Without agreement from the EU institutions I don't believe that we could unilaterally revoke A50 and carry on as if nothing had happened.

We could of course take it to the ECJ for a decision. Hmmm take the EU to it's own court....

Good luck with that then, and all the while the clock is ticking. Tick tock!

It's all hypothetical anyway because I can't see the UK trying to revoke A50 anytime soon. No deal is much more likely. 
Parliament has to approve a no deal Brexit.
As I understand it, Parliament gets to have a 'meaningful vote' over the deal, but dosn't hold a veto. It's what Dominic Grieve and his cronies tried to achieve, but he backed down and voted against his own amendment.

If on the 29th March 2019 there is no deal in place we will leave anyway. Amid much wailing and knashing of teeth no doubt!

Tick tock!

At the other end of the spectrum, if
Olly Robbins Theresa May decided to abandon BREXIT, I'm sure Parliament would have to approve the attempt to revoke A50, given that they overwhelmingly approved starting the process in the first place. It would be politically explosive, might lead to civil unrest and would certainly bring down the government leading to a GE.

Cue the return of Farage and if he organised UKIP properly they might do well in a GE amid such circumstances, given that 3/4 of English and Welsh constituencies voted leave and across the UK 2/3 of all constituencies voted leave.

That's hypothetical speculation though.
