In short I'm off to the Himalayas at the end of July where I'm cycling the Manali Leh highway. We'll mostly be camping and I'm told that the temperature will drop to just below freezing at night.

Rather then buy an expensive sleeping bag I’m planning to have a liner made for my cheap synthetic two season bag (EuroHike 250 I think). Essentially it will be a cotton liner that is fully lined with a silk/ cashmere mix material (two big pashminas!).

Any ideas whether this will be warm enough – I can get quite cold at night? I can always sleep in my clothes on for the really cold nights – down jacket, long johns etc.

you might get maybe another 5 degrees comfort rating with a liner.
whats your 2 season rated to?

as for liners, I can highly recommend these silk ones:
http://www.jagbags.co.nz/products.htm

you can also get cheaper cotton ones which you may be able to marry up with a silk one.

imho its best to buy a decent sleeping bag as there in nothing worse than having a poo night sleep. trust me i know i though on excersize once i would take a jungle doss bag it'il be fine nope far from it i had to be taken back in with mild hypothermia!

this is what i live by now

Prior
Preperation
Prevents
P155
Poor
Performance

I'd get a new bag. No questions. Are you anywhere New Mills? Magic Mountain factory shop usually has great deals on Mountain Equipment seconds and samples. Or there is the Rab factory shop in Alfreton.

Alpkit? CCC?

I think you might still be a bit chilly. Another option would be to buy another cheap bag and double it up with your current one. Probably about the same price as the pashmina idea and a fair bit warmer since a double bag traps a fair bit of air. You might need a bigger bag for the outer layer so that both of them loft nicely and so it might be a bit bulkier and heavier. It's also worth buying a down jacket so that you can hang around and admire the views in the evening and you can drape this over yourself inside the sleeping bag to add some warmth. Don't wear it though as you lose the insulating value of the bit you are lying on top of.

I'm very jealous! Was in Ladakh last August for my honeymoon and loved it!

Just get an Alpkit bag. Won't cost that much (a little over £100) and they're great. Have slept in mine on the Cairngorm plateau with 2ft of snow around. Kept my merino baselayers on 'cos I thought I might get cold, then woke up sweating in the night and had to take them off!

the Eurohike down 500 might be a better bet and is only £70ish.

Okay, thanks for the responses. Basically I'm in Dubai so decent camping kit is hard to come by – people over here are only interested in Prada and alcohol…

I can get a The North Face – Blue Kazoo down bag out here but it's quite expensive where psahminas are cheap! I quite like the idea of a second cheap bag though…

stevomcd – the AlpKit bag that I was looking at http://www.alpkit.com/shop/cart.php?target=product&product_id=16416&category_id=253 was GBP 150…

Oh and I'm leaving on the 30th july which kind of limits my options for mail order.

I take it you're not carrying the kit yourself?

cynic-al – Thankfully yes there is a support truck carrying kit. I'd like to think that I may be a little more organised if I was going it alone…

If you've got a decent down jacket to sleep in then you might be OK, maybe with a beanie on your head and base layers. Just make sure you have a big evening meal and all your base layers are clean and dry. The other thing that works well for trapping air and adding a season is a decent bivvy bag (not the cheap plastic bags), it also keeps the outside of you sleeping bag clean and dry, if you can get one over there.

Alpkit do international order, see link below

alpkit international orders clicky

Things may have changed since I was last out in Nepal over 20 years ago,
but there where plenty of shops where could hire expedition rated sleeping bags for the duration of your trip.
Saves you the expense of of buying one, and it saves a bit of weight/space in your luggage.

edd – think that's the same bag I've got, paid a bit less than that only a year ago!

If you've got a decent down jacket to sleep in then you might be OK, maybe with a beanie on your head and base layers. Just make sure you have a big evening meal and all your base layers are clean and dry. The other thing that works well for trapping air and adding a season is a decent bivvy bag (not the cheap plastic bags), it also keeps the outside of you sleeping bag clean and dry, if you can get one over there.

I worked with a Russian mountain guide who used to use one of those rectangular caravan sleeping bags which only came up to his waist. He wore his down jacket to cover the top half and a hat to keep warm at night. He slept on 1/2 a karrimat which he had cut and used to store loose as a liner to his pack. We looked right prats in our goretex bivi bags and lightweight tents next to Anatoli. He had been guiding for 20 years and had lead teams to Summit Everest and K2. I learned a lot from that man! I still take a full bag, thermarest and bivi though!

2 weeks is plenty for mail order.
Call and check if they've got it in stock first, if you're paranoid.