We had a combi fitted couple of yrs ago, and have the same problem. Had the engineers back several times, and they say its working ok and nothing can be done.
While using the sink / wash basin its not good but adequate, but having a bath is hopeless. You need to leave the tap running slowly to get hot water, so the bath takes forever to fill, and so has started to go cold before it fills up.
We use an electric shower, but the kids enjoy a bath, they like to splash around.
The only advantage we have found with the combi is, we have lost the tank in the bathroom, and so have more room, and do not need any water supply in the loft, so do worry about burst pipes in the winter, as much.