It might be worth giving Jena-Tec a call as they are the manufacturers and the D27 is listed as a current model:
https://www.jena-tec.co.uk/
There are number of online belt size calculators where you enter the pulley sizes and centre distance and it will tell you the length. The width should be easy to determine.
If it has multiple pulley combinations, work out the length needed for all of them as then you lessen the chance of an error.
Among many others, George Lodge (https://www.georgelodgedirect.co.uk/) has a good selection of flat belts.