There are trig points, which are almost all concrete pillars, although a few are 'bolts' attached to prominent landmarks such as church spires, water towers etc. A few hundred trig points have gone, but most of the 12,000 odd still remain
As well as the trig points there are 'lower order' OS marks used for more local surveying in between the main trig points. Some are metal flush brakes, as in my earlier photos in this thread. Some are cut marks, simply the OS crows foot carved into stonework, and some are bolts, a bolt head of a few inches dimater hammered in to a precise spot.
For trig points this is an excellent resource:
http://trigpointing.uk
For then lower order 'benchmarks', the flush brackets, cut marks and bolts:
http://www.bench-marks.org.uk
I got into this quite by accident. Its an interesting pass time in its own right, but its a superb excuse for an exploratory bike ride. Indeed, there's a cut mark I've identified I'm going to try and track down tomorrow.