Success!
I bought a new bell-box in the end and this weekend, thanks to some longer ladders, I successfully removed the old bell-box, disabled it and fitted / commissioned a new Texecom Odyssey 1E.
Connected the cables at the bell-box and put the battery jumper to the 'on' setting.
Flashing green left LED only, no blue strobe - excellent I thought.
Went down the ladders, into the control panel and connecte the other end of the cables - green rapidly flashing LEDs (expected as I read this is when the unit is coming out of hold-off mode back to an armed state).
A few minutes later - flashing blue strobe again; heart sank - the new bell box probably wasn't needed afterall (my fault for skipping the step below).
I renembered what you said about the spare wire - went back up the ladders and took the black strobe cable out at the bell-box - blue strobe went off.
Changed the cable over for the spare wire (had to strip the cable up the ladder with the alarm going off next to me!) and replaced the terminal cover + bell-box cover.
Went back to the control panel and worked out which spare cable was the corresponding end.
Blue strobe not showing anymore!
Used the test menu and confirmed the strobe comes on when requested and goes off when you turn the test off!
Finally reconnected the other bell box and tested strobe + bell functionality working as expected.
Performed a full set and test and all working as expected
Very pleased with the result - the new bell box is definitely far louder than the original too which is a bonus + I have another for Soares and repairs too now
Just wanted to also take the opportunity to thanks you both SO much for your help and assistance to diagnose the problem - well beyond the call of duty for a forum (amazing forum by the way); I couldn't have done it without your assistance
Thanks again for your help - it has been massively appreciated!!