does £60 get you much nowadays??
Not a huge amount for what you want - basically you pay money to get a smaller lighter bag, but just as warm. And something that going to fit in a 33L sack but warm enough for Spring is probably going to be down, which is going to be double the price. Frankly, don't think of trying to have 'one bag to rule them all' - its like having one duvet all year round. Instead, but a bag that fits your needs in winter, and then a lightish cheap bag for summer.
However, this is the time of year for sales, etc, so you might do well. Cotswolds havn't anything on deal in that price range on their flash sale, but for about £100 or less, you could get something from ME (Starlight III or even IV),
http://www.cotswoldoutdoor.com/mountain-equipment-starlight-iii-regular-sleeping-bag-d2232282? id_colour=133
plus some bags from Vaude (which I know nothing about), and Haglofs. They are all going to be synthetic. Remember that if get the Cotswold Explorer card (free - but yeah, its a data capture thing), you get some benefits, including 10% off your first purchase, which could make something like a ME bag a bit more doable. They also have their semi own brand Ayachuco Sirius 300 for £85 - for a bag rated to minus 6, thats not bad
http://www.cotswoldoutdoor.com/ayacucho-sirius-300-sleeping-bag-d2232257?id_colour=4524
Blacks are also having a sale, and the MH lamina 20 is £110 from them
http://www.blacks.co.uk/equipment/125014-mountain-hardwear-lamina-20-sleeping-bag-left-hand-zip.html
That seems to compact the smallest of the synthetic bag and is well thought of. I thought the foot was a bit tight for me, but try it out. And Rock + Run is doing the O version (so 4 season) for £116 - thats a very good price
http://www.rockrun.com/camping/sleeping-bags-synthetic/mountain-hardwear-lamina-0
Webtogs have a Snugpak (Ok, so not as warm as it says, but still) Expedition in a horrible 'Amber' colour for just £54
https://www.webtogs.com/en-UK/Snugpak-Sleeper-Expedition-Sleeping-Bag/34031wmp.htm?colour=Amber
and they have a TNF bag (rare this side of the pond) which is 3 season (down to minus 7) for £76 -
https://www.webtogs.com/en-UK/North...ur=Minus+7+Reg+Length++Striker+Blue+Zinc+Grey
. The US info is here -
https://www.thenorthface.com/shop/aleutian-20-7 and a decent review -
http://www.backpacker.com/gear/sleeping-bags/gear-review-the-north-face-aleutian-20/
Frankly, for the money, I'd vote for the TNF bag, and it should cover most of what you need, with the Cotswolds own brand coming a close second. But its not going to pack down
that small - if you want small, then its got to be down, and thats going to cost. Alpkit would be your goto brand, but their Skyehigh's have sold out at the moment - the 700 or 900 would have done very well for you at £150.
If you want the best for your money (or close), then try second hand. There may be bags coming up in the classifeds on this site, but Outdoor Magic's classifieds might even more choice, especially for down.
http://www.outdoorsmagic.com/forum/classifieds-section/10.html
And don't forget OM's bargain alert -
http://www.outdoorsmagic.com/forum/gear/bargain-alert/7436.html - stuff comes up all the time, be it flash sales, codes, factory shop sales, shops closing down, etc. And yes, I am the smug git who got a ME Classic 500 down bag for half price in a Blacks closing down sale..
And don't forget Ebay.
You never know what might turn up, but I suspect that to fill a 33L pack with something other than just the sleeping bag, your going have to up that budget a lot - the Rab 500 might do, but the cheapest I've seen it is £190. But if I was buying a bag, then the TNF one would give the most for least cash, but its bulky. Good luck, and let us know how you get on.