TrigpointingUK

There are quite a few of these. From those I've seen there are often 4 spires - I presume only one is the trig. I would have thought it would have been really difficult to sight on this from multiple other stations - the other spires would have 'got in the way'?

True. But the surveyors only needed to view it from two other points, so whichever spire was most visible from those points would have suited their purpose just fine.