Do you mean this @Scask 

https://www.ovoenergy.com/annual-boiler-service

Or perhaps here:
Boiler and heating cover – we’re here to help | OVO Energy

How to get in touch

Looking for help with your energy? Visit the OVO Energy help and contact page.

Got a boiler and home cover question? Call 0330 102 8905.

• If your call is urgent, use Option 1 for new faults and existing claims
• Want to make an appointment? Use Option 2 for our services department
• Got a question about your policy? Use Option 5 for customer service

National Gas Emergency Service: call 0800 111 999.

Other ways to contact us

• Write to us: OVO Energy, PO BOX 3129, Worthing, BN11 9QS

I don’t think I’d recommend the last option if your boiler’s broken down 

How do you contact OVO to report a boiler breakdown? Tried the contact number only open Monday to Friday????

Try https://www.ovoenergy.com/home-services/feedback#:~:text=Call%200330%20102%208905.&text=National%20Gas%20Emergency%20Service%3A%20call%200800%20111%20999.

Hey @User100, 

https://www.ovoenergy.com/boiler-cover you should be able to sign in to your account through this link. You can also call the team on 0330 102 8905.

All the options for OVO boiler and home emergency cover can be found here. 

I hope this helps!