Can you take the mechanism apart and see if it's worn or dirty? Many of these have plastic teeth that interlock to hold the head in the set position, if the shower head is constantly being moved or adjusted, these will wear over time, particularly if the joint isn't fully loosened before adjusting.
Leo we have taken it apart and there doesn't appear to be anything wrong with the whole thing. Its actually just the silver section where the shower head sits into that appears to be causing the main problem along with the heavy weight of the shower head itself. There aren't any plastic teeth or anything else to wear just the silver holder.
michaelm, might be an idea alright to just replace the fitting because we need to be able to remove and use the shower head when cleaning the shower itself.