The shipping rate cannot be determined at this time

I'm having trouble with 'Big Royal Mail'. It refuses to allow the use of 'letters' as a postage option. The estimator pop-up gives the following results for a 200g item costing £3.49:

Royal Mail 1st Class "Large Letter" (The shipping rate cannot be determined at this time)
Royal Mail 1st Class "Packet" (Great Britain and Northern Ireland only : 3.150 kg(s))£10.25Royal Mail 1st Class Recorded Signed ForTM "Large Letter" (The shipping rate cannot be determined at this time)Royal Mail 1st Class Recorded Signed ForTM "Packet" (Great Britain and Northern Ireland only : 3.150 kg(s))£10.95Royal Mail Standard Parcels (to £100 insurance) (3 to 5 days. Delivery to United Kingdom : 3.150 kgs)£9.24

Does anyone know why it's doing this?

Your package weighs 3.150kg. The Royal mail does not accept 3kg letters or large letters.
Under Admin-Configuration-Shipping there are a pair of settings for percentage/weight that govern how much weight is added to the product weight for shipping materials. e.g. 10% or 200 grams etc etc.

The default weight added is 3 units, in your case 3kg. Changing that to a more reasonable assumption will solve you problem.

All shipping modules also have a 'Display Invalid Shipping' flag, which if set to true, will always display the calculated values, making correcting errors a bit easier, but not if it does not have a rate for the weight supplied.

Thanks - I knew it was something simple!