A tripoint is a geographical location where the borders of three regions meet. The above dummy co-ordinates are at the West Sussex-Surrey-Hampshire tripoint. There is nothing hidden there of course, you will have to solve a puzzle for that.
I have taken five photos each from one county across the border to another. Its your job to identify those places. Go to each one and gather some numbers.
1) West Sussex to Surrey
Find the OS grid reference on the metal structure to your right
SU xxxxA xxxxB
2) West Sussex to Hampshire
Find William Smith's triangle, at the bottom of which you will see
BS CDE
3) Hampshire to West Sussex
Find the unhelpfully imprecise sign on the bridge behind you.
Count the number of times the letters F, G and H appear on the plaque.
4) Surrey to Hampshire
Find the phone number on the post to your left
077xx JKLMNP
5) Surrey to West Sussex
Look to your left and find the post box number.
GURS TD
You will find the cache at:
N51 0(B-K).RFE
W000 4(C-H).A(T+G)(S+P)