Had the same, one caliper seized. To be fair the £270 they quoted for a replacement caliper fitted seemed pretty fair for a main dealer.
However I fixed it with some brute force. A bit of heat on the bottom of the caliper and then a lot of decent hits with a drive punch (Link below) and hammer just above the slide pin got it freed.
https://smile.amazon.co.uk/dp/B000LFVNNM/ref=cm_sw_em_r_mt_dp_U_.5O4CbCWAF4YD
Water had got in and corroded it quite a bit. I didn't have new ones to hand, so cleaned up the hole and pin and gave it a good greasing and it's free now. I'll replace the pin and rubber at some point this year though.
This is the grease set I've bought for the using on the Lex
https://smile.amazon.co.uk/dp/B07886FP12/ref=cm_sw_em_r_mt_dp_U_l9O4CbB4BPJBT