Roof ladder hook/wheels (Bexley DA5)

Hello Everyone,
                               I am requesting a bracket attachments that can convert a step ladder into a roof ladder. I need to repair tiles in a tricky position on my roof, where I am not prepared to venture without the security of a roof ladder, I did make an attempt and slipped, so thought better of it.
If you have such an item you no longer need, please let me know and I will collect it directly.
Many thanks
Phi