A frost this late is not much of an issue as it tends to be only for less than an hour at 4am then the temps ramp up again. Unless you have really tender plants out...

Sorry, what exactly is a 'frost'? It's a term that I see very frequently from ppl living up north. I mean I can tell that the temperature sounds icy to me. Does it mean the temperate will get very close to 0°C? Thank you for explaining.

A frost Chatbud, is when the temperature falls below freezing and ice crystals form on the surface.
I wouldn't take any chances of not fleecing up, if a frost was forecast, even a ground frost.
You are very lucky not to get them where you are, but having said that, there are some plants that need lower temperatures before germinating, and also parsnips need a frost to give them a better taste before harvesting.

Thank you, Littlelegs, for explaining. Yes, while not having winter could be a blessing it also means there are various species of plants we could never have at the tropics. Eg. the Cymbidium and Miltonia types of orchids. Was told by a commercial grower the temperatures here are far too monotonous to induce flowering for these 2 orchid varieties. Cymbidiums and Miltonias are never found here for sale at where I live.

An air frost from April is not going to last long and will do little damage unless you have the likes of house plants outside. Fleecing is a waste of time in May for the hour or so that it can get to below 0C. Plants will get damaged more from wind or the likes of slugs, snails, vine weevils, caterpillars etc than a May frost.

Kildermorie, Whilst I appreciate that plants get damaged from all sorts of things, I would not take any risk's.
For the sake of fleecing up and the possible results of a frost, it would give me peace of mind, that my plants are protected. I have pasted an excerpt from the RHS website below.
"As a result of late spring frosts summer bedding plants and tender vegetables, such as potatoes and tomatoes, may suffer from leaf scorch, browning and even total plant death"

I couldn't agree more littlelegs:)

You will do more damage in covering plants new growth with a fleece than the frost will do. There is a massive difference between a frost in December and one in mid May. If you have such tender plants then they should not be out so early.