Planted out forty seedlings and eleven were left by morning, which slug remedies actually do anything?

The single biggest change is not a remedy at all: plant out later and bigger.

A slug can destroy a seedling in one night because the whole plant is within reach and there is very little of it. The same variety planted out three weeks later, twice the size and with a tougher stem, gets chewed and survives. It is the same slug population and a completely different outcome.

So:

Pot on rather than planting out at the first opportunity. The impatience is understandable and it is what costs you the tray.

Harden off properly so the plants are not soft when they go in. Soft growth is the preferred food.

Plant out on a dry spell if you can choose. A warm wet evening is a slug convention.

Do not plant out everything at once. Half now, half in a fortnight. That way one bad night costs you half a tray rather than a season.

Beans and courgettes in particular are worth growing to a decent size in pots. Lettuce is genuinely hard because it never gets tough, and for lettuce the physical barriers and the trapping matter more.

On the remedies, the honest ranking is below, but do this first: it changes the size of the problem rather than fighting it.

Having tested most of them over several years, roughly by how much they actually did:

Going out at night with a torch: genuinely effective and nobody wants to hear it. Two or three nights in a row, an hour after dark, especially after rain. You will be shocked at the numbers and the difference is immediate. It is the only method that removes them rather than deterring them.

Beer traps: work, with caveats. They do catch a lot. They also attract slugs from beyond your beds, so sunk right among your seedlings they can make things worse locally. Site them at the edges, not in the middle. Emptying them is unpleasant.

Pellets: effective. If you use them, use the iron phosphate type sparingly and according to the packet rather than scattering generously - more is not better and the packet rate works.

Copper tape: works on a pot, useless on a bed. It has to be a complete unbroken ring with nothing bridging it. One leaf touching the ground outside and it is a doorway.

Eggshells, coffee grounds, sand, ash: did nothing measurable for me. They are widely repeated and I would not spend time on them. Ash and grounds also stop working entirely once wet, which is exactly when slugs are out.

The longer game is to change where they live, because you are not fighting the slugs in your bed, you are fighting the ones in the metre around it.

What helps over a season or two:

Remove the daytime shelter next to your beds. Slugs spend the day under things, planks, pots, dense weeds at the edge, long grass against a bed, an old bag of compost. Clearing the strip immediately around a bed does more than anything you put on the soil.

Water in the morning, not the evening. A bed watered at dusk is damp all night, which is when they feed. This is a free change and it is worth more than most of the remedies.

Encourage the things that eat them. Ground beetles, frogs, hedgehogs, birds. A small pond is the single most effective thing if you can have one; a log pile and a gap under the fence are the cheap versions.

Accept a sacrificial zone. A few plants at the edge that you do not mind losing take pressure off the rest.

And the reassurance: a night like yours is usually the worst it gets, because it combined freshly planted, freshly watered, soft seedlings on a warm damp night. Change two of those four variables and you will not see a repeat of that scale.