Frost more likely in sheltered spots?

I think it's because at this time of year at night, when there is no cloud the ground cools very quickly, and so the air immediately above the ground also cools quickly if it is still and not moving (sheltered), and so can locally drop below the frost point. If there is a bit of breeze or air movement, it will stay warmer above that spot of ground because the air itself is not below freezing, and as it's not the same bit of air continuously over that spot it doesn't chill like standing air would.

That's not very clear, but if you think how quickly ice forms on a still pond or a puddle, compared to how long it takes on moving water like a stream, that gives you the idea.

But I wouldn't worry about it too much, esbo. Just remember that frost hardy plants won't be bothered by it, and anything frost tender you need to keep protected till the danger of frosts has (hopefully) passed. Which I find is about mid May in the Midlands.

I am hoping global warming allows me to trim that back a couple of weeks

Hmmm, well, just to let you know, I was scraping frost off my car this morning

It really is getting to know your patch. I live fairly high up and can be subject to high winds and longer frosts that a gardening mate who lives just a mile away but at a much lower level.

Really? I am not far from you, Nottingham, I think it was about 4C last night not 100% sure I would be
surprised if it was freezing.
The weather site has yesterdays weather and confirms 4C for notts

I don't think there is any chance of frost at this time of year where I am.
It is close to the centre of Nottingham, the heat from the surrounding housing must make it a degree or so warmer.

Be your own weatherman! How is it that the national weather forecast, the local weather forecast and the Ceefax weather forecast are all saying something different, though the information all comes from the same source (the Weather Centre)?
One of life's mysteries!

I have this thermometer with a sensor on the end of a cable which I hang out of the window so I can see the temperate on the digital readout.

It is a long long time since there has been a frost, probably February I'd say.

Mind you it is close to the house and windows sill level so maybe not as cold as ground level.

I am 99% certain I have seen the last of the frost this year.

Yep...it really is about getting to know your own patch!
My neighbour across the road has a sheltered potager and his last frost date is mid May

On the other hand, our exposed field/ potager has had frosts as late as 9th June

The temperature can vary quite drastically over a very small area. In an average sized garden for instance, the lowest overnight temperature can vary as much as 6 or 7 degrees.

If you measure the ground temperature, it could be 0 and yet measure the temperature 3ft above ground and it could be 4 or 5 degrees. Therefore if you have a sensor that you stick out of a window, say 5ft above ground level (and obviously sheltered by the house) the temperature in the same garden could be 7 degrees or even more.

Ground frost is the main danger at this time of year, not frost in the air.

As others have said, frost is a very local thing. It depends on a huge number of factors (some tricky to explain), the most important being:
Lack of cloud, so that heat can escape easily into space.
Lack of wind to mix the air up, as mentioned above, allowing cold air to sink towards the ground. Cold air is denser than warm air and unless there is wind to mix it up it will be colder near the ground than a few feet higher up. This effect is the opposite to the decrease seen in temperatures at greater heights when the air is thinner and therefore (usually) colder, so the tops of hills will have later frosts than lower regions.
"Frost pockets" are areas which are prone to cold air settling into them so tend to occur in valleys and at the bottom of hills.
Exposure to wind chill can freeze vegetation due to evaporation of water, which has a cooling effect.
Towns and cities and areas near brick or concrete walls are always warmer as these materials act like storage heaters.
The coldest time of day is shortly after dawn, because although the sun has risen and started to warm the air, the heat loss is still higher than the heat gain so the temperature continues to fall for a while before levelling out then rising.

Covering your plants even with thin material like fleece can protect from most of these, but if the actual air temperature falls too far they will not do so. That becomes more and more unlikely as the days get longer and the sun gets higher in the sky, but there are plenty of other factors (such as the source of the air mass covering the area and of course your latitude) which are important too.

There's a brilliant little gadget you can get that looks like a usb stick. Its a temp data recorder and you can leave it in various places in your garden overnight to record the changes in temperature and then plug it into your computer and it will give you a data log of the temperatures. Only costs about £5 - £10

John that is such a good idea, that good I have just bought one of the Bay. A slightly more expensive version as I will use it for work too, but then again the tax man pays.

Thanks mate.