You will find all the answers within a few metres of the square, mostly on the south and east sides. You may well cross the road five or six times so take care. Two hours' free parking in the nearby multi-storey (or, while it's closed, the car parks over the river) will be ample for this and some nearby caches.
For all year answers take last two digits only, eg 2015 becomes 15.
At start coordinates the second year is A.
Go to County Hall. Number of dormer windows is B. Make sure to look on all 4 sides! It was erected in C and restored in D.
Now the rest you can find yourselves...
The year the king whose name is in white on black near a MARKET PLACE sign died, is E.
CG Barber's year is F.
Counting from the top, Outdoor Pool is number G of ten fingerposts.
One for Angry Birds fans... the Bad Piggy is number H of 5 counting from the left.
John Roysse signed an indenture on which day in January? That's J.
Mabel's son was Archbishop of Canterbury for K years (second date minus first).
The church bells were cast in L.
There's no longer an M...
A pulpit was given to the church in N.
Obelisk Lamp Standard dates P - Q.
Victoria Statue Q - R.
Abingdon Cross c. S - T (PQRST can all be found in the same spot - under cover but not indoors).
Cache is at N51° (S+K-G). (B+D+E+F+H+J+N) W001° (Q-C+A). (A+B+C+D+E+F+G+H+H+J++K+L+N+P+Q+R+S+T)
Yes, that last bit is one of each plus an extra H.