Hello Ian and Julie,
It's important for the shrouds to be pretty tight, as Geoff said, to protect the mast from bending the wrong way, to avoid shock-loading and breakage that results from slack rigging, and to allow the sails to set correctly. It's not necessary to adjust the shrouds in order to raise and lower the mast on the 235, though (assuming the tensions are appropriate. It's well worth buying a tension gauge for accuracy.)
However, I too used to worry about the tension needed on the mast-raising tackle in order to fit the forestay pin. I then realised that the jib pole/sprit that used in the mast-raising operation is too short and consequently pulls back on the halyard, adding unnecessary tension that fights against you. The solution is simply to disconnect the pole from the mast once the mast is upright and before tensioning the tackle to connect the forestay. (The pole is no longer needed at that point.) Likewise, when lowering the mast, set up the pole in the usual way but then disconnect it from the mast in order to tension the rig and disconnect the forestay. Then ease the tension, reconnect the pole to the mast and lower away.
Without the pole pulling on the halyard, the tension needed to fit/remove the forestay clevis pin isn't excessive.
My boat was used on the Broads by her first owner, who had an A-frame fitted just as you have. I still have it and used it when I first bought the boat. It does allow the mast to be lowered pretty readily for passing under bridges (once the boom's off, obviously).
Hope that helps.
Anthony Russell
235/02 Sea Wyche