We have the Campaign (and Personal Campaign Page) option appearing within Contributions and I believe it is impacting on automatic receipting to supporters. Does anyone know where or how to delete this campaign extension.
CiviCampaign is not an extension but a component (ie it's part of the core), you can enable/disable it from (administer>system settings>Components)
and disable it.
This being said, I'd be surprised it's the issue, lots of install have civicampaign enabled and working email receipts