the final update on this saga
rear disc replaced
rear caliper overhauled with new pistons [ scuffing marks on old ones] new seals fitted
new brake fluid
new pads [ metal gear]
and still the grinding noise would come and go so today i fitted genuine triumph brake pads [actually SBS] and on measuring both sets found the metal gear pads were 1mm thicker so no wonder the pistons would not retract all the way in as there was an extra 2 mm of brake pad to compensate for !
oh well its all good now and a lesson learnt