It failed again so I tried the adjustment suggested by Dick Wells. I have had limited opportunity to road test but so far its now working perfectly. I didn’t find removal/refitting too much of a problem and the following tips may help anyone else doing this.
After removing the single securing bolt it is necessary to depress the brake pedal to relieve the spring pressure on the switch so it can be wiggled free from its upper locating tab. On refitting, the brake pedal must be depressed to allow the top of the switch casing to engage in the top locating tab and the peg on the brake pedal arm to engage in the slot in the switch casing.
With the switch refitted and bolt tightened the brake pedal will only come up as far as the opened up ratchet adjustment mechanism of the switch will allow; to complete the adjustment the pedal should be pulled up against the resistance of the switch (clicks will be heard as the ratchet mechanism takes up the adjustment).
I did not find it necessary to make the JB weld repair suggested by Dick - even though wear was visible and the switch is original with 100,000 miles of service. It seems that adjustment alone adequately compensates for wear.
Thanks everyone for the advice - the piggy bank is still intact!!