In this paper we analyse the requirements of performing parallel transaction-oriented simulations within loosely coupled systems like ad hoc grids. We focus especially on the space-parallel approach to parallel simulation and on discrete event synchronisation algorithms that are suitable for transaction-oriented simulation and the target environment of ad hoc grids. To demonstrate our findings, a Java-based parallel simulator for the transaction-oriented language GPSS/H is implemented on the basis of the most promising shock-resistant Time Warp (SRTW) synchronisation algorithm and using the grid framework ProActive. The analysis of our parallel simulator, based on experiments using the Grid5000 platform, shows that the SRTW algorithm can successfully reduce the number of rolled back transaction moves but it also reveals circumstances in which the SRTW algorithm can be outperformed by the normal Time Warp algorithm. Finally, possible improvements to the SRTW algorithm are proposed in order to avoid such problems.