Contact Zen sales and explain the situation and see if they offer any easier solutions.
I'd cancel the work/Zen line completely, and migrate the home/Plusnet line to Zen with a SIM provide FTTC.
The Plusnet/ADSL line will convert to Zen FTTC 10 working days after placing the order, with little to no downtime.
You could arrange the ceasing of the work line separately, to end on a date of ur choice.