Display checkboxes instead of sliders to show price ranges and allow the user to select one more more price ranges. Use price intervals of £10 so that there are about 4-5 price ranges visible.
If the range sliders have to remain, do not change the initial minimum and maximum values once the slider is activated, Instead, allow the full range to remain visible and accessible so that the user can reset the prices.
Once the user applies the filter they cannot 'undo' their action, i.e. they cannot decrease the minimum price or increase the max price. E.g when range is £20 - £25, it cannot be decreased to £15 or increased to £30. The only way to reset the price is to backtrack (visit another page) and revisit the page.
Estimated loss from this usability issue (using conservative estimates):
Estimated daily traffic: 50,000
Estimated percentage of visitors who use the price range filter: 2% = 1000 visitors
Estimated percentage of visitors who fail to find the desired product or abandon task and don't purchase: 2% = 20 visitors
Estimated average order value: £10
Estimated daily loss of potential gain: £10 x 20 visitors = £200
Estimated yearly loss of potential gain: £200 x 365 days = £73000
I am absolutely certain that this usability issue alone is causing a minimum yearly loss of £73000 (Actual figure is likely to be significantly higher).