not strong enough for corolla plug leads

vasaline.

Twist through 90 degrees to break the seal then pull.

silicone grease, or wrap some rubber round the tips of your pliers. Self amalgamating tapes good.

Thanks to all. Would not vaseline or silicon grease interefere with the quality of the connection between the plug top and the metal clip that grabs it? Also I would be grateful if you would educated a novice on exactly what 'self amalgamating' tapes do? many thanks.

Nope

It's like electrical insulating tape but you stretch it about 50% & it then amalgamates (sticks) to itself. It's fairly soft & rubbery so you can grip things with it.

The message from "jw 1111" contains these words:

No - not in the slightest. Lubricant or water displacing grease is commonly used on electrical connections and provided the connector isn't shoddy to start with never causes a problem.

It comes on a roll and is like thick insulating tape. The difference is that it's supplied with a disposable backing strip rolled with it. To apply, you unroll it, peeling off the backing as you go and stretch it while wrapping it. After a few hours the rubery stuff of which it's made will have melded with the next layer in, and so on. Eventually it'll have "self amalgamated" into one solid rubber layer. Dead clever, and in the right situation very useful.

Ford used to recommend applying silicone grease to the lead fittings on the zetec engine.

silicone grease would be better electrically than vaseline.