Negative head means that your shower head or tap is above the top water level of the cold water tank.

For negative head we recommend that the cold is mains fed and you double boost the hot feed.
For simplicity of operation a radio remote is a great solution.

SP1 (Manual Pump) £83.98
SP1U (Double Boost Upgrade) £55.00
Radio Remote £15.00

Total £153.98

You could go to Screwfix and buy a negative head pump for between £279.99 and £489.99 or a Shower Power Booster which will deliver a solution for half the cost of a traditional pump.