yes its worth it, IF they are priced ok (and even if they are not, heres what i did:
get old WC to introduce you to the customers
after the first clean, include with your "your windows have been cleaned tickets" a bit of a newsletter introduing yourself, personally, i included details about my family aswell, making a point that i am a familyman etc etc, tell them how often you clean, and what advantages you have over the old WC'er (this will include you use a specialist soap etc etc), point out you will keep the prices the same and review after 6 months
if they are too cheap, put the prices up after 6 months
i brought 90% of my work in, and its worked great for me, i paid 1X the amount, you do hear of people paying 2X or even 3 X the rounds worth, persoanlly i think this is a touch high
go for it, 400 quid isnt much, BUT DONT FORGET you are just buying the goodwill, they could drop you anytime