I found something interesting. You have choice of only 750 watt and 1000 watt proprietary power supplies - proprietary due to motherboard connectors 20 pin and 24 pin. Since you need stronger one, the only choice is 1000 watt. Original model is NPS-1000AB-1, another one which looks like the same and used for Dell Precision 690 (also 1000 watt with 24 pin and 20 pin connectors) is ND285.
So if you live in US, this is the places you can get it. NPS-1000AB-1 - Impact for $134.95 new,
Genuine Dell ND285 Precision 690 1000W at Amazon, used and refurbished in US $125 for used and and 129.99 for refurbished.
However it looks like you living in Europe (using £), so I took a look at England suppliers, found one, it is looks puzzling here due to the difference in price for two units which looks very similar: ND285 for $120.75 USD £78.92 GBP and NPS-1000AB A for $224.25 USD £146.57 GBP there is no direct link to the parts, so use search, both refurbished with 30 day warranty. May be there are more supplier choices over there, now you know part number, so go ahead.
Lets us know if need more help.
