No race license needed for TTs, you just need to be a member of a club which affiliates to the CTT - Addiscombe does, so you are
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The Open bit in TTs means that they are open to riders from other clubs rather than being a club members TT (which is what the evening 10s are). Clubs promote one or more TT through the CTT (governing body) for anyone to enter, these are called opens and everyone doing their bit ensures a huge calendar of races are run up and down the country.
Oh, and I'm sure you'll improve your position. From memory that was your first 25, I think first Open TT, and the conditions were horrendous. You did well to compete and finish, out of the 120 riders who were meant to take the start line only 49 did.