No, there's no way of importing receipts and allocating them. 

However, you could consider making the accounts auto-allocate (see https://desktophelp.sage.co.uk/sage200/professional/Content/SL/EnterNewCustomerAccountPayment.htm) if you just allocate receipts to the oldest invoices.

Unfortunately I can't use the auto allocate function as the payments don't settle blocks of invoices.  They may settle 500 invoices within 2000 invoices.

I was hoping there would be a template where I could upload the receipt against an invoice number - I would create the template from the remittance the customer sends.  Instead of entering 1 receipt value I would have a csv file with 500 rows, a row for each invoice with a GBP value - the total of the 500 rows would be the receipt amount.

Codis Excelerator - https://www.codis.co.uk/excelerator for Sage 200 have both a Sales Ledger Cash Allocation and Sales Ledger Receipt Allocation modules that should help you achieve what you are trying to do.