The original battery is 36 volts and I think 9 Ah. You can have any Ah, but best to use 9 or more to ensure good enough performance. I don't know if your cells are vertical or side by side formation, side by side can be recelled if you have the ability to solder them, but vertically stacked cells can be much more difficult.
Really good NiMh cells are very difficult to obtain now and expensive when they do exist. The move to lithium closed most of the good production lines for the D cells which your battery needs. Trying for packs in Britain is even more difficult, but
this page on the American Batteryspace site has packs which might have the formation yours uses
Best bet is to find out the price of a new one through your nearest Giant dealer, since that could be cheapest. I'm sure they are available since the NiMh ones for the much older Giant Lafree are still sold by Giant. Here's the dealer finder page for you, enter your location and then click the "Hybrid" option for the e-bike dealers:
Giant dealers
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