Quick update on this.
Used the overland express option as noted above. I was told it was probably best to use APC as they cover hifi amps over £200.
Amp collected & scanned yesterday so told the buyer to expect it today as it's on a 24 hour option. Nothing arrived, tracking number showing nothing on the site. Contacted overland, guy says there was an issue with the scanner though states his end it does say the amp was collected??
No contact from the guy after numerous emails, no tracking, no amp.
Buyer is being patient but I have a bad feeling about this. Having dealt with many a courier, something feels decidedly off in this case. Really hope i'm wrong otherwise £400 in the bin.