They range in price massively from £23 to £255 on some websites with more well known brands coming in around £65 (Hella) and Denso (£91) on AUTODOC. Bosch is about the price MX5Parts are quoting for the expensive one.
Nothing to lose trying the £24 one and worst case can always then go with Denso from another supplier and still be under the MX5 Parts price. Denso is a massive supplier of parts to automotive industry.