Osmo colour revive.....

I would just mix a little of the red dye you use with your conditioner. Every time you wash, just leave it on for a few mins.

Thanks for your advice! Is there a deepish red semi that gives a good colour that you would recommend? I have to admit to using a permanent red recently because my colour needed the lift from the developer. I have used Directions dyes before but would they work on a deepish red base? xx

Permanent reds are brown bases, so you're kind of undoing whatever lift you were doing ... I'd just use a colour remover and go with a deep red semi, personally.  Better colour and tone and lasts longer, too.

For deep red semis, try Blood Red or Hot Lava by SFX.  Hot Lava's an orange-based deep red, and Blood Red's a pink base.  You can mix them together for a ruby tone.