You are very welcome. To help, do note that all the .20 barrels on all the AIRGUNAID SP5 .20 HUNTERS that I have seen over the years were not Milbro manufactured, but 15mm diameter Walthar Lothar barrels, so it follows that the DG .20s were fitted with the same.
This in mind, why not contact Walthar Lothar direct, on the off chance that they still manufacture a similar barrel for other models, or maybe even have some old stock in their spares bay (it's got to be worth a shout )? Atb: G.