Gardtec 800 Series Error Code?

[adam.cruickshank@xxxxxxxxx (Adam Cruickshank)]

To cut a long story short, I was given a code with the alarm when I
bought my house and it turns out it's the master code.
The misses managed to blindly press buttons with the code while trying
to disarm and now I can't arm the thing anymore.

The sequence is:
Displays funny lettering (see end)
Key in code, displays SE
Press yes
A short continuous chime, displaying E with 3 vertical bars
Continuous chiming (de dah de dah), displaying funny lettering
Have to key in code again
Displays Un
Continuous chiming (ne nah ne nah), displaying funny lettering
Have to key in code again
Displays funny lettering

Funny lettering (like at t1, but without the bottom part of the t).
|_  |
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Does anybody know how to fix this?  Or what it would take to fix it?
A live in Thetford, Norfolk if anybody is local.


Thanks in advance