Hello, I hope the following assists you.
As the engine management light has come on then the engine management has detected a fault, this is why you have the loss of power. From your description the most likely cause will be due to a misfire caused by an ignition system fault, either a spark plug, HT lead or ignition coil. Start with checking the spark plugs for any sign of electrical arcing on the outside and the electrodes for damage, if one spark plug electrodes are black then this is the cylinder affected by the misfire, also check the HT connector/leads for any damage or corrosion in the connectors. A misfire will be worse as load on the engine increases due to the higher voltage and longer spark duration required.
If all appears OK then ideally to confirm the misfire have a diagnostic scan carried out to retrieve the fault code causing the light to come on, if you want to do this on a DIY basis then an example of a diagnostic tool is http://www.ebay.co.uk/itm/NEW-MaxiScan-Diagnostic-MS309-Code-Scanner-Reader-CAN-OBDII-OBD2-EOBD-UK-STOCK-/171224683835?pt=UK_Diagnostic_Tools_Equipment&hash=item27ddc9553b
The above applies to a petrol engine as you mention the lack of power when you "give it more gas" a misfire on a diesel is likely to be an injector concern.
Let me know if I can advise further and I'll reply as soon as I'm able to.