How can I fix the outer door lock on my Dyson CR01 washer?

My Dyson CR01 (at least 10yrs old) washing machine keeps asking me to "Close outer door and press start" even though the door has clicked home and seems shut as normal. I've tried resetting the codes using the trick of holding down the soil type/temperature/on/off buttons etc. and this brings up the diagnostic code, but doesn't help.

I rang Dyson, who said they no longer repair these machines. I could try an independent repair man but I'm worried it'll cost a lot of money.

Has anyone managed to fix this problem themselves? I'd be grateful for any advice.