Concrete Mix for cavity

Hi All

I know that there a quite few threads about concrete mix ratios, but I can't seem to find one that tells me what ratio I need to fill the bottom of the cavity.

I know it's got to be a "weak" mix, but any suggestions on what ratio?

Also, fitting some drainage pipes and usually use Wickes Silicone Lubricant to grease the ends. Any other alternatives that I could use from around the house? Used to use washing up liquid but read that this could damage the seal.

Thanks

8:1 lean mix ratio.

fairy liquid will not damage the seals, if it did then all drain seals will have degraded due to rinsing fairy down the drains. it just dries that's all and makes removal later on difficult. silicon remains viscous.

Thanks noseall

Saves me a trip to wickes!

Better get out mixing now, while the suns shining!!