Allowing for possible differences in W.C.'s between the U.K. and here n the U.S. and interpolating your question, the first thing to check is the simplest. No "oomph" in the handle sounds to me like the chain connecting the handle to the flush valve. This is the valve that opens to begin the flushing process. The chain can become corroded and break (happens regularly). Take the top lid off (don't drop it... things get unimaginatively expensive with small mistakes). Move the handle while observing the end of it inside the tank. If it is broken, often one can repair it with a paper clip tying the two ends until you can buy a new one (they're inexpensive). If that's not the case, or if your model is different from the one I describe, best to have someone look at it. The system is fairly simple and little can go wrong that can't be fixed easily... usually...