I found this page - not sure if it's the same scheme?
https://pages.ebay.co.uk/nectar-rewards/programme.htmlOn refunds, it says:
2.2 Refunds:
If you return a purchase for a full refund, we will re-issue the voucher(s) which has been used for this purchase;
The voucher expiry date will be extended by 30 days if the original voucher expiration date is less than 30 days from the date the return is processed. Otherwise, the expiry date will be the same date that was displayed on the original voucher(s);
In the case of a partial refund, voucher(s) cannot be re-issued and points will not be returned.
Seems to suggest that partial refunds aren't a thing, though I could be reading it wrong.