Thanks for the info.
Now that I have one of the keys on my keyring and have put the other on the spare keys ring I'll have another go at it.
I'm sure my neighbors were happy (as I was) to know the car has an alarm...

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95 E300D gave away
77 300D, 227k, station car
83 300CD 370k, body gone away to the rust gods, engine is in a Yota pickup, going strong
89 190E 2.6- 335k, no more
79 VW FI Bus- 154k summer driver
59 VW Beetle ragtop- 175k
12 VW Jetta- 250k
74 MG Midget-78k