I changed BCH in my Trezor to BTC using the wallet coin splitting tool and sent it back again to the same wallet using a new BTC receive address rather than another external wallet like eg Electrum using Shapeshift exchange. But the BTC didn't arrive. Have I lost the coins or are they just lurking in cyberspace waiting to be found again ?
If you sent BCH to your Trezor BTC address or vice versa, you can use https://trezor.io/recover-coins/ to recover your coins. The only exception is if you send BCH to a BTC 3... address (segwit), since these are not supported by Bitcoin Cash.
To convert BCH into BTC or vice versa you need to use an exchange.
Double check your addresses as it is very easy to mix them up when dealing with splitting then Shapeshifting. You can check your transactions and wallet balances on the respective blockchains such as Blockchain.info for BTC and Blockdozer.com for BCH. In other words, you still may have both BTC and BCH as the transfers might not have transpired.
You can't convert BCH to BTC using the splitting too. You can only claim BCH that you got as a result of owning BTC when Bitcoin Cash forked. So, after splitting, what you end up with is new Bitcoin Cash, not new Bitcoin.
If you sent that BCH to a BTC address, what you actually did was send Bitcoin Cash (not Bitcoin). This situation is already covered by joho's answer.