Replace key barrel in WB fuel cap

[rolesyboy]

I decided to put my Mini on a single key.
Changed the various barrels to match.
I decided to get the Wilmott Breeden cap to match so pinged off the brass circlip, filed down the tumblers to match the rest of my car and then realised I don’t know how to persuade the circlip back on. There is a small hook in the centre that I think aligns with a tiny spring. Little room to work.
Anybody got any ideas on how to get the circlip back in
Thanks. Mark