Helen,
Tip = Get a friend with a small hammer (or socket bar etc)
Get him / her positioned so he is ready to tap the solenoid.
(safety) = Get van in neutral gear !
(safety) = Put handbrake on hard !
(safety) = Double check van IS NOT IN GEAR !!
Turn key to get that click and hold the key in the full clockwise starting position.
Get the friend to hit the solenoid (or even starter motor) try it once or twice.
If it then starts to turn the engine, you have positivity identified a duff solenoid.
If it does not,
Once started it is usually ok for the rest of the day - and better the next day.
borrow a battery of a friends car, make
VERY VERY sure you connect it up the right way around !!, you know RED to +ve terminal on battery and black or bare braided wire to -ve connection. Try it then.
( A low or failing battery can cause the solenoid to click but not have enough ummph to turn the engine over. )
As has been said, poor connections on the big fat wires around the battery or around the solenoid can cause this too. (safety) = When checking it is ESSENTIAL that you take off the earth connection off the battery whilst around the solenoid and starter with spanners. Believe me, you don't want to see the results of a short from a live wire to the body via a spanner !! Take care !