I am currently running a dynamic analysis for an abpve ground pipework according to PD8010. This section of pipework is known to vibrate when transient events happen in the line. Line surges have been obtained by means of hydraulic analysis.

However, how do I determine the rise time from the surge analysis to input into the DLF generator in order to run a response spectrum analysis.

See page 8-20 in the Applications Guide where the Water Hammer example is discussed. This may not be 100% applicable, but this should give you a starting point.

Thanks for the prompt response. I have seen the relevant section in the Application Guide, but it does not clarify how the rise time is determined.

Is there any general guide on how this can be worked out from a transient flow graph as well as duration of impulse load?

For impulse loads, rise time affects high ferquency response and duration affects low frequency response.
I would expect the timing of the event causing this imbalance would be a good starting point for your rise time. Does a closing valve cause the event? If so, then the valve closing time would be important.
I might also suggest that, for any single run of pipe, the rise time could be related to the time it takes the (instantaneous?) surge to round a bend into the run. That is - (pi*bend radius/2)/(speed of sound in the pipe fluid).