Thanks for your reply.
However, regardless of how large my paper loss is on my short call, I still wish to hold this position until expiry, as it is part of a wider strategy.
So if I get assigned on my short call when it is deep in the money, I would realise a large loss. However my question is, if this happens, could I just instantly sell the same calls with the same strike & expiry? As the option is deep in the money, the calls would have high intrinsic value and so the credits from that would surely almost cover the losses from the assignment? And so then my synthetic short stock position would continue on ... (?)