Can I change my gateway number for making tax digital as I used the wrong gateway reference number

Yes you can, @mewie50.

We can still change your gateway reference number in your Making Tax Digital (MTD). All we need to do is to disconnect it first then reconnect it back to update the reference number.

Here's how:

1. Go to Gear menu and select Account and Settings.
2. In the Advance section, click the Pencil icon for Making Tax Digital.
3. Once there, disconnect the MTD then reconnect it.
4. Update the correct gateway reference number.

I'm also adding a helpful article to learn more about submitting VAT return using MTD: Authorise QuickBooks to Interact with HMRC - MTD for Small Businesses. 

If there's anything else I can do for you, please let me know by commenting below.