Apparently, candidates are allowed to "observe" and indeed ask certain questions of the staff.
Every day is a learning day.

I guess the OP is takling about exit polling, or general local party election polling people.

These people cannot be inside the polling station, but to know where that starts is up to the presiding officer and might be a line within a room not the whole building.

They are allowed to ask you your polling card number and your choice of party, but it is up to your if you give them the info.

Inside the polling station only the registrar can ask for your polling number and they will give you the polling candidate sheet.

Candidates, with rosette can attend polling stations.

People working on polling stations can also talk to voters, but generally not until after they have voted.

THere are plenty of little laws around all this, if you are concerned go and ask for the registrar or presiding officer and ask them.

It was interesting that a candidate would be allowed inside the polling station and to both ask question's of the staff AND be allowed to look at who had voted (not who they had voted for) but, this theoretically would allow that person to chase up any stragglers ?

Obviously fair play has to be both in play and seen to be working but, it was certainly a first and I've been voting for 40 years.

Yeah, the register is available, I would argue that if a candidate is in a polling station chasing up, then that candidate is not out knocking doors.

Any worthwhile candidate would know where their vote is months before an election and be out ticking them off on the doorstep, not checking in the register.

I think he may have been sweating on the possible outcome !