Hello fellow UK unicyclist,
I did the very same to my learner uni, which I suspect was probably even the same unicycle from ebay. I ended up upgrading the seat, tyre and pedals without realising I should have just bought something new.
You’re lucky, as now there is an awesome cheap uni on the market. It’s the Here is a review of the unicycle (it has a few other upgrades but these are surplus for the time being, honestly) from a fellow forum member which you may be interested in. This should clue you in on everything about the unicycle.
And just in case, i’m sure someone else will back me up and recommend this uni.