Hi dallond,
I also have a Heritage 550ES with a working alarm, the factory code is *(star)1111, then press 1 to activate the alarm or 2 to deactivate it. I'm not in the van at present and this info is from memory, so apologies if its not quite right. if all fails, go into the small cupboard where the fuse board is and next to it is the black alarm box. There are two plugs on the side of it which can be removed if you cant turn the alarm off.
I do have the manual for the alarm so I could copy it and send it to you if you let me have your address?
Hope this helps and email me if you want a copy.
Cheers,
Simon
simonmac@tinyworld.co.uk