As alluded to above, the workaround is to reduce the element voltage. This can be a more satisfactory solution than changing the element if the power can be varied to allow for other loads being used at the same time etc. So while changing the element is an option, and probably the best option if you are not electrically experienced, it's not the way I would go personally. (Personally, I would avoid heating water with a generator-fed immersion but I know there are times when it is useful to do that.)
First point, don't reduce the power with a phase-angle controlled device (dimmer, triac regulator) as the generator voltage will probably go wobbly. A transformer is what you need. A variac will give infinite control all the way to zero but at great expense. An autotransformer with suitable taps is the conventional approach. If the element is rated at 1kW at 240V and we want to reduce it to 800W, the voltage required would be 240 x sqrt(800/1000)= 215V. Suppose the generator gives 230V under full load. A 1kVA autotransformer with 220, 230, 240V taps, with the 240V tap to the generator and the 220V tap to the load, will give 220/240x230= 211V, or 773W. This will allow some room for the unavoidable loss in the transformer.
But a general-purpose autotransformer of that size is still £100+.Because the desired reduction is a small fraction of the supply voltage there is another transformer approach that is cheaper; the smaller the reduction percentage the cheaper it is. This is a bucking transformer, which is just a stepdown transformer with a primary to suit the supply and a secondary rated for the actual load current at a voltage equal to the desired reduction. The primary is connected across the supply, the secondary is connected in series with the load but in antiphase*, so its voltage opposes the supply.
In the above example, we want to reduce the 230V generator output by 15V, so we choose a transformer with a 15V secondary. The element resistance is (240)²/1000 = 57.6Ω making the current 215/57.6=3.7A. The required bucking transformer is then 3.7 x 15 = 56VA or greater. So the problem can be solved with a transformer like this for £23, wired with its two secondaries in parallel:
75VA 15V chassis-mounting transformer at RS
I will stress that if you are not electrically skilled I don't recommend 'having a go' at this, as you will need to consider fusing, thermal protection etc. But if you can build this safely, I contend that it is the cheapest and easiest solution that works perfectly but still allows you to use the element at full power on a shoreline in the future.
*Where does the 'missing' power go? Well, the transformer draws its magnetising current from the 230V winding, but the power flow is actually from the 15V side back to the 230V side, in effect feeding the sliced-off bit of power back to the beginning to lessen the load on the generator.
