WorldPay plug in - how to set Merchant's reference

I have HikaShop working with the WorldPay plugin and correctly sending 'test' purchases from HikaShop to WorldPay.

WorldPay uses what it calls 'Merchant's reference:' to store a reference to the transaction. At present it either sends the Contents of 'Default Description' or 'Use Single Product Name in Description' (ie in System > Payment Methods > WorldPay Business Gateway

If you use 'Use Single Product Name in Description' it uses the chosen product (hardly ideal as a 'unique' reference).

If you purchase more than one product then it switches to Default Description.

Is there anyway to set this to something more useful (and unique!) like the HikaShop Order ID?

Thanks in advance for any suggestions

>>>>>> EDIT >>>>>>>>>>>>>

OK - I have now discovered it does actually send through Order ID as 'Merchant ID' but instead I am seeing it list 'Decsription' when I make the transaction (and foolishly thought this was the uniqueID).

This seems to be fine. I need to look at my WorldPay settings on the WorldPay website rather than HikaShop.