Close, but no cigar.

                          using (var context = new PICSEntities(Config.model.SqlServer))
            {
                var q = context.TransactionDetails.Where(t => t.Parcel.GoodsType.Description == "Rough")
                    .Where(t => t.Parcel.ParcelInventoryType.Description == "" +
                    "Inventory")
                    .GroupBy(t => new { f1 = t.Parcel.RSReference /*, f2 = t.TransactionHeader.TransactionType.Description */ }, (key, g) =>
                      new
                      {
                          Parcel = key.f1,
                          BeginningInventory = g.Where(x => x.TransactionHeader.TransactionType.Description == "Beginning Inventory").Select(x => x.Cogs),
                          Purchase = g.Where(x => x.TransactionHeader.TransactionType.Description == "Purchase").Select(x => x.Cogs),
                          Transfer = g.Where(x => x.TransactionHeader.TransactionType.Description == "Internal Transfer").Select(x => x.Cogs),
                          Sale = g.Where(x => x.TransactionHeader.TransactionType.Description == "Sale").Select(x => x.Cogs)
                      });

For the avoidance of doubt, I'm trying to get a table consisting of a line for each Parcel and the total Cogs for each TransactionType (Beginning Inventory, Purchase, Transfer and Sale on the same line but in a different column. It seem to put a collection in each cell where I was hoping to get the Cogs.