Why does mixer smoke and smell like burning cloth while still running

There are several reasons why the machine has reached this state. The first being the failure of capacitors and/or other components on the speed controller board at base of the unit. Either you will have single speed running or dead machine. The brushes may have reached their limit also. If motor is running really slow in all ranges, a dry bearing may be ready to seize.

In all cases, the unit can be carefully disassembled and rebuilt and with readily available parts or sent out to a professional. Personally, I like to DYI - Kelly.

Very common problem on those is old capacitors on the pcb. Sorry I cannot help you where to buy them but search for supply in the UK should produce results.