Standard advice: spend time reading
http://www.seat61.com/Poland.htm#.Usxv__uP-Ng
To get to Torun for noon on Saturday 11 Oct, and return Sunday at 16.30, you can use the overnight sleeper Amsterdam-Cologne-Warsaw as far as Poznan and then use a Polish "express" train (you return via Kutno which is further East). You could leave Chester as late as 11.35 Friday to make the connections and be back around 15:30 Monday: Chester-Cologne via London and Brussels is a simple journey. You could, if you prefer, start Chester earlier and overnight in Berlin.
If that arrival is too late, you would need to spend Friday night in Poznan (a good choice if you don't speak Polish) which means leaving Chester on the Thursday (London-Poznan is just possible in a day, but if I could afford the time I'd spend Thursday night in Brussels or Liege rather than spend all day on the train.
As far as tickets are concerned, you book with DB (
www.bahn.com, or via their UK call centre)
- a London Special for London-Brussels-Cologne
- a EuroSpecial Poland for Cologne-Poznan-Torun (with the appropriate sleeper upgrade if travelling overnight - you have to specify "regional train only" for the last section, but don't worry, this is just to show the fare, the ticket is OK on "expresses")
Chester-London International is a special connecting ticket from Eurostar Telesales
Finally, you need a compulsory Seat Reservation for the TLK express from Poznan to Torun, which you can get at Poznan station on the day for a few Zloty (write your request down in case staff don't speak English).
I've not used a TLK since reservations became compulsory, someone else on here may have more info.
Booking 3 months in advance you should be able to do the journey for a bit over £100 each way plus sleeper berth or hotel costs.
If you are travelling alone, you might find going via the night Harwich-Koek van Holland ferry cost-effective, as their single cabins are quite economical, but you'd still need the hotel in Poznan. I'll give details if you want.