Increasing calcium intake

i've just looked on that well known auction site and its on there and is suitable for birds too.

He probably recomended it as he is more used to dealing with reptile pets than chickens (unless he is a chicken vet)

We used to keep aviary birds and some of the supplements they had were also suitable for reptiles, small mammals

The calcium supplement we used to use for the birds was calcivet. that comes in either a powder form or a liquid form that can be added to water or fruit pieces (we used to add it to the water as if they didnt eat the fruit it was a waste of time putting it on, at least we knew they had to drink the water)

Someone with more chicken expereince may have better chicken alternatives.

In severe cases you can increase their calcium by giving hens a pinch of limestone flour on a grape or similar - and again this is found usually in the reptile section (for turtles and tortoises etc. as they need it for their shells).

Both Nutrobal and Calcivet are basically calcium as in calcium chloride (which easily dissolves)/limestone flour/chalk /etc (not liquid calcium - which doesn't exist) plus Vitamin D3.  Davinova C I'm not sure about.
The key is in the vitamin because calcium is hard to absorb and vitamin D is the regulator. This is why children deficient in vitamin D (the sunshine vitamin) develop rickets even with a calcium rich diet and chooks with lots of grit around can have soft shelled eggs.
Both products are used for small birds and safe if expensive for a lot of chooks.
You can buy calcium chloride cheaply on ebay (http://compare.ebay.co.uk/like/140492287249?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar&adtype=pla)  at a much cheaper price and get vitamin D from the same or local shops and mix your own. If unsure, you can get human tablets for example  (http://www.ebay.co.uk/itm/Calcium-200mg-Vitamin-D3-90-Tablets-UK-100-/150612247781?pt=UK_Health_Beauty_Vitamins_Supplements&hash=item23113084e5#ht_2763wt_1390) and crush/dissolve. Again cheaper than small pet medications.

The layers pellets have oil added but a drop per bird once a week won't hurt.  I used to sneak it into their porridge

I used limestone flour but it didn't help with the soft egg issue unfortunately