Hi,
If you have been making taxable supplies since you have been registered then you should have been accounting for VAT on these supplies even in the absence of a VAT number. The guidance below explains how to account for VAT in this scenario:
Who should register for VAT (VAT Notice 700/1)
You will therefore need to account for the VAT on these supplies. We would recommend contacting the customers to see if they are willing to pay the VAT over to you although they under no legal obligation to do so.
You can see guidance on cash accounting here:
Starting to use the Cash Accounting Scheme
Thank you.