How much are the arrears at the moment?
Are repossession proceedings imminent?
In that case I would suggest that you pay the mortgage and keep a record - it will not change the position on the arrears due to you
As a starting point be patient - facing prison your ex may well decide to negotiate with you so wait until you see the outcome of that.
Alternatively he himself may apply to vary the order - which will open the matter for your to apply to vary it yourself.
All of these will be easier than applying to vary the Order until the next time arrears accrue
If you have decided on a Judgement Summons then try that first
It is possible if he continues to pay erratically and accrues arrears
That will be a Form A - but wait until the enforcement has failed first
The Judgement summons
My apologies - it is a D62 not a D63 that you need first
You have sufficient evidence (the plane tickets) to show that the non payment is a choice and not a necessity - that is all that you need
You wait until his arrival in the Uk and arrange for him to be served personally.
You will need to explain to the court the difficulty that you face is service and ask for the time for service to be abridged (shortened) to seven days
You submit it to the Court that issued the Order.
You will need to
1. Ask the Court for permission to "abridge the time for service" - making it shorter
2. arrange for the paperwork to be personally served on him as soon as he is in the UK