I Decided to change my uncapped service to capped after 4 days. I bought the new LTE capped at a Telkom store and brought it home. BTW add two days for 'System Meltdown' at Telkom then got through to 10210 (finally).
I explained the situaton and was told that I had to keep the modem as it had been taken out of the box. I replied that it was impossible to assess the modem/service without doing exactly that and furthermore, the local Telkom shop had taken the new modem out to put in the SIM and activate it.
I asked whether the account was now cancelled and was told No - but the request had been put to the cancellation department (11.30am 18th April). Let's see if it goes through.
BTW - rather get your modem from a Telkom store than online because you can cancel the account by taking it back to the store.