I have a function to attend mid-august for several days.
The hotels in cambridge are pricey and would be looking upwards of £1,000 and don't want to pay holiday prices.
If I were to attend cambridge and return to london and vice versa, what train tickets would I be looking at buying?
Would I have to buy individual tickets for the day? There's only open return tickets as far as I can see.
Look on Trainline.com, put in the dates and times, read the page, ask for the cheapest ticket. If you haven't got a railcard, it will cost about £50 for an anytime day return. If you are travelling from Kings Cross or St Pancras, there are about 4 trains per hour, 2 Thameslink (utter rubbish) and at least 2, maybe more northern Line (relatively luxurious compared to Thameslink, but fewer seats so often more full).
As @threeportdrift says, an "anytime return" you can make one journey to Cambridge and one journey back at any time on the same day. The "anytime" part means "any time on the same day", as opposed to an "off-peak return", which would allow travel only during the "off-peak" time of that day.
An "open return" allows travel outwards on one day and the return on the same day or a different day (I think) up to a month later.
Your other option is to look at buying "advance" tickets. Those are likely to be cheaper, but are only valid on a specific train. (So if you miss that train your ticket is void and you'll need to buy a new one.)
If you are eligible to get a railcard and don't already have one, you might find buying one is worthwhile. A railcard costs £30 and is valid for a year. They give a one-third cost saving on most tickets, subject to some exceptions.
Which searches for split ticket fares as well as through fares for you.
I’ve chosen a random date in August (Tuesday 15th) and asked for a return to Cambridge at peak times (leaving at 08:00, returning at 17:00) and trainsplit has quoted me £16.00 for two advance tickets (advance tickets mean you MUST catch the booked train and only that train, but are much cheaper due to the lack of flexibility).
If you were to use these for a week (5 days) then your travel would only cost you £80 compared to the ludicrous hotel prices!