Zen have been really good over the last years I've used them and we only use them for work but I'm finding their price (£45 per month) hard to justify over £22.50 with PlusNet as I'd save £270 per year by moving.
What service(s) do you current have with Zen, and what service(s) are you looking to take from PlusNet.
£22.50 is PlusNet's monthly charge for unlimited fibre broadband, assuming, like the majority of people, that you're in a PlusNet "low cost" area. The same service can be had from Zen for £35.40/month for
Fibre Unlimited 2 without Zen line rental, or £30/month for Fibre Unlimited 2 with Zen rental at £15.44. Zen's FTTC minimum contract period for FTTC is 12 months, whilst PlusNet's is 18 months.
Zen's ADSL pricing may well be revisited within the next few months, as it has been a while since those products were refreshed.