If you've a Wickes in your area, buy their own make multi purpose compost, comes in 70 litre bags. I'm wondering how on earth you're going to tip all that into your planter, that's a lot of compost. I'd be inclined to also buy some proper John Innes No. 3 to mix in with it though... choose the one made by Westland, its better than the others. This comes in smaller bags because its much heavier than multi purpose, but it'll give some oomph to the multi purpose stuff. Suggest 7 bags of Wickes and 3-4 bags John Innes... or 8 Wickes and 3 JI.

Thanks - that was quick! I'll get my son to do the 'loading'!
I did wonder about doing a mix as you suggest, but I've read some bad reports about the Wickes GP compost. Also I would the John Innes be peat free? I'm a bit concerned about the environmental problems associated with the extraction of peat. Thanks again

Well the traditional 'recipe' for JI was loam, sand and a percentage of peat, but they may be substituting that with something else by now. Even if there is a bit of peat, it will be very little - I remember when I'd buy several 80 litre bags annually, year on year....

Yes, I recently bought a small bag of 'Verve' JI No.3 for some pots. I thought it was peat free, then read the small print and discovered it had 20 per cent peat.! I'ts a minefield out there!!! Anyway, thank you for your help. When I get it all done, I might post a picture!