In reply to sheelba:
Hathersage is in a steep sided valley, and the topography is inevitably smoothed out by the resolution of the forecast model; if I recall correctly, the Met-office currently runs the UM forecast model at 5km resolution, but don't quote me on that.
This means that any relief rainfall effect is bound to be underestimated. This is the same pretty much anywhere, when I lived in California it was noticeable that the NOAA model underestimated rainfall in the western Sierras and overestimated in the Eastern Sierras/Owen's Valley (the Sierra Nevada range is very steep and narrow).
If you have the (human) resources you can correct for this using expert judgement for a specific location, which I suspect is basically what MWIS does. I very much doubt they can run a forecast model themselves, so they probably get model output under contract from the Met Office, ECMWF or both. They then make an adjustment for specific areas of interest in which they have some expertise (in this case, mountainous regions of the UK).
When you click on the forecast map from the Met Office website, it's basically just model output with no human intervention beyond QC. If you want a very specific forecast from a trained forecast you can get that, but you have to pay (it used to be about GBP20 to phone a forecaster).
In summary, the Met Office output, which has to cover the whole country, does not have the location specific adjustments that an expert analysis will provide, and it's this gap that the MWIS provides.
When most people grumble about how the Met Office 'got it wrong' (and my how they grumble), it's because they think they've watched a forecast after the news. They haven't; they've watched a general synopsis of the ENTIRE country in allocated time of 90s. In practice this 90s is always cut to closer to a minute, and at weekends the footie results squeeze it down to 30s. Often this time squeeze is happening while the forecaster is on air and talking; the producer is shouting down the ear piece to wrap it up. You know that other constant bitch, why are regions like Scotland often skipped; want to guess why that happens?
30-90s to cover the entire UK, and the public expects them to get the nuances of whatever valley, mountain or beach that they happen to be interested in; that expectation is not just ignorant, but downright narcissistic.
So, in response to the OP, I would trust the MWIS more. That's not because the Met Office are crap - MWIS wouldn't be able to do what they do without them - but because the goals, parameters and constraints of the different products are entirely different.
Will (trained forecaster, now researcher)