[dionne]

Had a great weekend at the allotment,very lucky with the weather i tested PH of my soil it is around 7.2,some of the beds will need to be more acidic  than that,i want to try and stay as natural and organic as i can with the plot,i have read that putting sawdust helps to add acid...is this true? Any other ideas??? I will be digging in manure next week.

[WG.]

No sawdust.  Double, triple, or quadruple your manure application.  You can also mulch on surface.

You can add peat if your conscience permits.  

Remember to leave a bit unmanured for root crops.

[Trillium]

If it is at all available to you, powdered yellow sulphur can be scratched in as well. I use it on my rhododendrons and they're quite happy.
Note: this isn't the same as aluminum sulphate.