The original main halyard consists of a 4mm kevlar line. Kevlar does not cope well with being pulled around small blocks. It frays and then snaps without warning usually choosing a critical moment to do so.
 
A suitable replacement line is a 4mm dyneema 12 strand rope. This is both strong and is gripped well by the halyard cleat. You will need a length of 12 metres, whilst a slightly shorter length will do it is more cost effective to cut 150mm from the top of the line at the end of each sailing season and the overall rope will then last for many years. The cost is around £2 per metre. Choose a colour that distinguishes the main halyard from the downhaul,  gnav and gennaker control lines.  Fitting the new line is extremely simple. Remove the hoop from the Xenon to allow the mast to be lowered, then remove the mast from the Xenon. This is usually done with the dinghy on its trolley or road trailer and having an assistant sit on the front of the dinghy to keep it stable (prevent it tipping backwards). Climb into the Xenon, ask the assistant to release the forestay and then gently lower the mast towards the rear of the boat, taking care to keep the mast in the centre of the boat. Then remove the main pin from the mast step (the heavy duty plate into which the mast heel is fitted).
When the old line is removed take the opportunity to clean and apply silicon lubricant to the masthead block. With the mast laid across a couple of chairs, with the halyard track upper most, thread the new line into the mast track via the entry hole sited just above the gnav bracket and push it along the track to the masthead sheave.  Use a needle or pin to coax the end (heat sealed) of the line into the block and around the sheave. Pull the new line through and tie a bobble on the end.  Tie another bobble on the other to prevent any accidental pull through on a windy day.  Job done.