Shower has no hot water

Our mixer shower was fitted initially in May and within a few days, the cold water supply from the shower stopped and it was just boiling hot all the time. We thought it might have been an airlock because the valve/bar controlling the shower temperature kept getting stiff like there was pressure behind it. A second plumber came out and connected the cold water supply only to the mains, but now we have just cold water in the shower and no hot. The basin in the bathroom has both. I've asked the plumber why we had only hot in the shower and now only cold water, and he has said this is because the cold water supply is stronger than the hot water supply and is overpowering the hot. He said the pressures need to be equal for both temperatures, but this doesn't make much sense to me as we had hot water at a good pressure and now cold water at a good pressure- we have just never had them together. He says the only solution is to get a pump fitted for hot water. Beginning to think we are being conned into parting with even more money, quoted £400-£500 for a pump. Please help!!