I finally got the topmast plug out! Phew. Method was:
One. Buy a big drill from Screwfix.
Two: abuse my power drill and drill a hole through the middle of the plug.
Three: use an original 1980’s hot air paint stripper to push air through the plug as often as possible.
After a few days the plug came out. It is now fitted properly.
I also added a burgee halyard cleat:
Plus I’ve made a tarpaulin pocket to hold the anchor so it is ready for use next to the tapernacle.
Update: here’s the picture:
The anchor is a Guardian – a cheaper version of the aluminium Fortress. It was bought for a 21′ 1100kg boat so is a bit oversized for the Wanderer. However it is very light and I own it so that’s what I’m using. I use old climbing rope for the warp as (a) I’ve got 300′ of it and (b) it is very stretchy. The mount works nicely. I didn’t get the shank mount in quite the right place but it fits ok.